Take a look at this, very simple explanations that are straight forward, and a good visual comparison between the lenses and their effects.
Ultimately the availability of different lens choices allows the photographer to decide how to portray their subject, the more lenses available to you the more possibilities afforded in those taking situations. Everybody will have a particular feel about what they like in focal lengths. Ask yourself about the application and the goal you are seeking to obtain with your photographs, and the choice of lenses will be obvious. Also be wary about the lust for speed, it'll end up costing you an arm and a leg, and is a hard thing to give up when you have a taste of it.