It is often said that photography is a matter of compromise and in the world of lenses that is true as well. No matter how good a lens is, it can have distortions and other oddities that it will introduce into a photograph that it taken. Thought most of these are very slight in some situations they might become objectionable and are correctable.
The types of problems can be vignetting, lens distortion (both pin cushion and barrel) and chromatic aberration for example. You'll have to visit each of the above links for explainations and I have included a few graphics here as examples.
One of the neat features of most of the Adobe family of products is their "Profile Based" lens correction. Adobe has a large database that contains many of the problems associated most of the popular lens. However, not all lens have profiles built for them.
There is hope however. Adobe has supplied us with a tool and instrucitons to create our own profiles should the lens you are using does not yet have a profile built for it and is called The "Lens Profile Creater". It is free for download from Adobe Labs.
Here are some further instructions from another site that are worth reading.