TortoiseSVN is a free Subversion-based version control system that makes it easy for software developers to keep control over their projects.
The program comes with all the usual version control features. You can check out the files that you need to work on, commit any changes when you're done. There are options to update your files with changes from other developers in your team, and assistance in resolving conflicts where you've both changed the same lines.
You also get extensive tools for displaying the differences between various files: changes you've made, the differences between two revisions, even a "difference with blame information" report that shows you every change, who made it, and when.
And because TortoiseSVN tracks every change to every file and folder in your Repository, it's straightforward to revert back to any previous version you might need.
Most convenient of all, TortoiseSVN installs as an Explorer extension, providing all its options from a file or folder right-click menu. And so it's not tied to a particular development environment, or even source code, necessarily - you can use the program to track changes to any set of files that you might use.