Cogito is popular interface to the Git version control system. It is designed to be comfortable, easy to use and present gentle learning curve for new users. It is not by far as full-featured as "raw Git" is, but although the plain Git interface went a long way since the first days, Cogito still provides a simpler and easier to learn interface. If you want to start to use Git and are considering Cogito, the best way to go about it is to first learn Cogito, then pick up Git commands if you need to do something extraordinary.
Cogito is now deprecated and no longer actively maintained.