Git is a distributed revision control system with an emphasis on speed. Git was initially designed and developed by Linus Torvalds for Linux kernel development. Every Git working directory is a full-fledged repository with complete history and full revision tracking capabilities, not dependent on network access or a central server. Git's current software maintenance is overseen by Junio Hamano. Git is free software distributed under the terms of the GNU General Public License version 2.
So, that's what Wikipedia says about Git. And it is of course all true.
Unfortunately, that definition is all very clinical. That perspective is not gonna do much to help you know why you should even bother with Git.
To answer that question, you need to know what Git does for you. You need to know what problems Git solves for you and how to get yourself out of a pickle when you find out you've been using your hammer all wrong. You need to be able to learn while using Git, to fix mistakes you discover along the way.
To answer why to use you want to use Git, read the the Git Primer.