gvim is actually a part of vim distribution. It's really a superset of vim - it provides all the vim functionality and then some. gvim stands for GUI Vim.
Some of the additional features include:
Because gvim runs as in X11 or in Windows it provides native methods of clipboard use. That is you can select stuff in X and expect it to be copied on middle mouse button click in other window. Basically gvim is just a prettier version of text mode vim.