The most important thing with regards to comfort on a bike is to be sure that your
bike fits properly. Many women end up with overly padded shorts and a big fat thickly padded saddle instead of with a bike that fits properly. No saddle will be comfortable if the bike is too big, or set up incorrectly. It is important to find someone who knows about fit and specifically about women's fit and get the bike set up properly before making other changes. In addition to being more comfortable, a bike that really fits will also handle better than one that is improperly sized. It isn't always easy to find someone willing to take the time, but when you find a shop that will, give them lots of business and send your friends there! Go to shops during non-prime hours for the best service. You won't get a salesman to spend an hour letting you try different saddles on a Saturday afternoon, but you might on a Tuesday morning.
It's certainly getting better, but in even in recent times, most production bikes were built proportionally for the average man. Of course, the average man tends to be larger (taller, broader shoulders, etc.) than the average woman. So women, especially smaller women, may have a much more difficult time finding a bike that fits. Using the old guidelines of sizing a bike by straddling the top tube may result in a bike with a top tube that is too long, since many of these smaller bikes have shorter seat tubes, but the top tubes are left at the same length as larger bikes, so the bike is just not scaled proportionately. Compensating for too long a top tube with a shorter stem may change the geometry of the bike and it will not handle as well as it should.
Empirical evidence has come to suggest that many women are more comfortable with a shorter top tube - stem combination than men. Originally it was theorized that this was due to women having longer legs and shorter torsos than men of the same height. Statistics have proven otherwise. But despite the similar proportions, many women still felt stretched out on bikes on which men of the same size felt comfortable. There is no one definitive explanation for this. Some have proposed that women may bend from the waist while men pivot more at the hips, which would explain why two riders with identical torso lengths might still want different top tube stem lengths. Georgena Terry has observed that women tend to sit further back on their saddles than men, which she believes is due to different distributions in muscle mass. Again this could lead to that stretched out feeling.
A riding position that leaves the rider too stretched out can cause saddle pain, hand pain, shoulder and back pain. It is not necessary to run out and buy a new bike right away if the top tube on your current bike is too long. Using a shorter stem on a this bike may give you a more comfortable reach. Very short stems, less than 40 mm, are available, but may have to be specially ordered. TTT makes a handlebar called the Morphe, which sweeps back slightly to shorten the reach. The difference is very small, but it may be enough.
Some shops use a fitting system called the Fit Kit. The numbers generated from the Fit Kit are just guidelines and may not work for everybody, especially women, since most of the original data was collected for male racing cyclists. Some shops may have an infinitely adjustable stationary bike. These are often used for ordering custom bikes, but may also be used to fine tune an existing machine or a new production bike. When buying a new bike, you can trade stems, bars, saddles, cranks etc. up front with little or no additional charge. Changing these things after you have purchased the bike can be very costly.
Front loading stems - those with two bolts on the front - make stem changing easy and painless, since you won't need to remove brakes, shifters and bar tape to swap a stem. Most threadless stems are front loading and some quill stems are as well. I definitely recommend this type of stem for fine tuning fit. They also make travel easier if you have to disassemble or pack the bike.
Adjustable stems are also be available to help you and the shop pick a perfect length stem the first time, rather than the expensive trial and error method of buying different length stems repeatedly until you find the right size. Unless your current bike is a really poor fit, you should be able to make a few relatively inexpensive changes to improve the fit. Then when upgrading or buying a new bike, use what you have learned to buy a bike that fits better.
I've talked a lot about reach (stems and bars), but crank length is another place where many women have poor fitting equipment. There are loads of different formulas for recommended crank length. I, personally, ride a crank longer than recommended by many of these formulas. I can't really recommend any particular formula, since I ride with longer cranks than they suggest for me. But if you have difficulty spinning or feel you can't get enough power, a different length crank might help. This can also get expensive.
It is important to ride your bike and make small adjustments until you achieve a perfect fit. Change, even for the better will feel strange at first. Drastic changes can even do more harm than good.
Of course the spending power of women seems to have enlightened some production bike makers, and many now offer women specific models in well proportioned smaller sizes, with narrower bars, shorter reach, shorter cranks, smaller wheels, etc. It's getting better, but it's still not great. Even those production builders with women's specific models don't offer them in their top of the line bikes! And one often pays a higher price for a women's specific model.
Some builders tried to shorten the top tube by increasing the seat tube angle, which then may place the rider uncomfortably far forward over the pedals. This forces the rider to use an adapter in the seat post to get the saddle back, which counteracts the shorter top tube. A steep seat tube angle may be good for a time trial or triathlon, but is not comfortable for longer distances, recreational riding or touring. And if it is true that women tend to be more comfortable sitting further back, then this is really counterproductive.
A sloping top tube has been used by many manufacturers to achieve a shorter seat tube and more standover clearance, but this leaves the top tube length the same as that for a larger bike, so the smaller rider still feels stretched out on a somewhat out of proportion bike...
Several manufacturers have started building bikes proportionally sized for smaller riders to specifically address those needs. There are several different approaches. Terry pioneered women's bikes with a small 24" wheel in front and a 700C or 26" wheel in back. Many smaller frames are built with two 26"or 650C wheels. Using two wheels of the same size simplifies what's in your spare kit.
To avoid confusion, let me state that by 26", I am referring to 559mm bead seat diameter. This size wheel is most commonly used in mountain biking. Thanks to mountain bikers use of very narrow rims, and a few tire manufacturers willingness to make narrow, slick tires for this size, these wheels can be used to build smaller bikes with proper proportions. Several manufacturers make 1 inch and 1.25 inch high pressure slicks which are very nice for loaded touring or casual riding. A bike using this wheel size is quite versatile, given the large variety of tire sizes available.
By 650C, I am referring to wheels with a bead seat diameter of 571mm. These wheels have found their way onto many triathlon bikes. These wheels are also occasionally referred to as 26" wheels, which is why the bead seat diameter number is so important. Tires for these two different 26 inch wheel sizes are not interchangeable, and it is very important to know which one you have. This wheel size is less versatile than 559, since only very narrow tires are made in this size. If you have rough roads, want to carry a load, or just feel more comfortable on wider tires, be aware of this. These tires are also not as easy to find in all bike shops.
700C is of course ISO 622, and is still the most common wheel size for road bikes today.
See also:
Although most road bikes have carbon forks, frame materials differ (mostly based on price). Bike frames are typically made of aluminum at less than $1500; between $1500 and $2200 the frames begin to show some carbon, particularly in the chainstays and seatstays; over $2000 buys a full carbon frame. Not all carbon is alike, however – precision engineering and specific shaping of the tubing affects stiffness and comfort. Steel and titanium bikes are more of a niche these days – high-end “ti” bikes give a supple, smooth ride; steel is heavier, but loved by purists for its lively ride.
or 
level) components at best. This is very frustrating for the very serious competitive female triathlete, because she will need to upgrade to the top-tier (
Comfort mountain bikes are very similar to hybrids in that the rider position is very upright and comfortable. They are intended for very light, non-technical, non-aggressive trail riding because the components are simply not beefy or durable enough to withstand the abuses of hard trail riding. 
A hybrid is a great choice for anyone who needs a “do-it-all” bike but isn’t a serious rider. Technically, a hybrid is a cross between a road bike and a mountain bike. Hybrids have 700c wheels, and run wider tires, usually with just a subtle tread or inverted tread, for better handling on dirt paths... 
The “urban assault vehicle” of the cycling world. Meant for utilitarian use, city bikes are lighter than mountain bikes, and heavier and more durable than road bikes. Some have road-style drop handlebars, some flat mountain bars, depending on rider preference. Typically city bikes have 700c wheels (road wheels) but run wider tires such as hybrid, touring or cyclo-cross tires. 
