The biggest difference between the MacBoo Pro and MacBook Air line is portability. The Air is lighter, thinner, "cooler" to look at... but the down side is that they can be significantly more expensive when compared to a comparably equipped machine. They also have smaller storage capacities and lack any user upgrade options - save a few third party SSD. And they lack the internal optical drive.
But, depending on what you have, and what you are considering upgrading to, they can provide a modest performance boost.
More specifics would allow us to better answer your original question.