components
 

i dont know much about bikes and up to now trying to find best fit bike but same time would like to know if some components better on one then others so have idea what i am getting and if down the road may be changing some.
could somebody please compare components of " Canondale Synapse alloy 5", "Scott speedster S20" and " Trek 2.1 "
All of them come with 105 but different wheels,tires,brakes,cranks ....

Brakes and cranks are an easy place for bike makers to cut corners and save money. FSA and Tektro are decent alternatives. Bars and stuff are generally inconsequential. Bontrager, for example, is just Trek's own brand of accessories. Of the 3, I like the Scott because it's the only one with a true full Shimano setup.

Components?
http://www.youtube.com/watch?v=OIh78GiTqrE&t=50s

what about wheels and cranks ?

Ignore them.

Seriously.

Getting a bike that fits is what you need.

After you settle on a bike, come here and ask about
any components you have a question about.

When you test ride (and don't buy a bike without one) make sure you hit
a pothole. That will tell you what the ride is like.

A lot of guys buy a frame that is one size small. After a couple weeks they
want to go fast and try to lean into it and find that it doesn't feel right.

I already mentioned cranks. As for wheels, there's generally nothing special about the wheels that come on low to mid range ($500-2000) bikes. They're generally heavy and machine built. If you get big into the sport, you'll probably make them one of your first upgrades. So, as mentioned above, don't worry about them.

Most entry level bikes are just that, to get you into the sport and get you experience. So comparing components on those bikes really don't do much good.
As a beginner, just pick an entry level bike that fits well and comes from a reputable maker, don't worry too much about components. Ride the thing till something breaks. By then you'll probably have enough experience to have an idea of what you want to replace it with.

^ With all due respect, I disagree somewhat. If someone thinks they might get more involved in the sport, they might want to choose bikes that are more easily upgraded. For example, running Sora makes it more difficult to upgrade to the 10 speed groups because you'd have to do a big chunk at the same time.