Compared to a regular road bike, women’s triathlon bikes are designed with a different geometry. The seat tubes of a triathlon bike are inclined more forward with 75 to 78 degrees of inclination and are lower on the front end to provide a time trial position. Due to its steeper seat tube angle, the quadriceps are emphasized more. This spares the running muscles so you can conserve some energy for the run later on.
Ordinary road bikes on the other hand are designed to corner, climb, and sprint. Their seat tube angles range from 72 to 74 degrees and are set up to cater for common riding positions. They are more upright and less aerodynamic so they require more muscle power to move them forward.
However, if you do have a decent road bike, there is no need to panic and go out and get a new bike. You can adapt the one you have. A road bike can be transformed into a triathlon bike by adding a forward seat post and aero bars.
Of course it is great to have both, but if you are just starting this method will do just fine. You can upgrade later when you know you like it and when the time is right.
The wheels are really important to the bike as they have a major effect on the speed. Most people have a pair of training wheels and a pair of race wheels. When you are starting out, do not worry about this at all- just get training!
But if you are a little more advanced, use a pair of cheaper wheels for training but buy the best pair of wheels you can afford to race on. (This pair you do not take out in all weathers, through the mud and pot holed roads!)
A pair of aero wheels for racing will enable you to cut through the air much better while also achieving higher speeds, in timed events they will help trim off the seconds. Many women are worried that they will look silly if they enter a race without all the latest gear. Do not worry about this at all. People enter events all the time on mountain bikes, borrowed bikes from friends or their old high school bike. Yes there will always be a bit of bling at the event but there is nothing more satisfying than cycling past a blinged up cyclist on your old fashioned cheap bike because you are simply fitter, leaner and stronger. Focus on your training- the better bike will enhance your performance but it will not make up for poor training.
In the end the athlete still has to do the work! Yes there are very pretty bikes in cycling magazines but these are pro cyclists who are sponsored. Do not compare yourself to these people. Get real, get training and get down to your races on any bike you can find!
The other key to cycling is bike position. Once you have chosen a bike it is a really good idea to get a specialist bike fit. Bike training takes up such a large proportion of your training I would almost say that if the bike shop does not provide a fitting service- go and find somewhere that does. It is imperative that you are comfortable.
This is the difference between chatting to some local experts and getting proper service to buying one $100 cheaper on the internet but not getting the service. I would go for service every time.