Mercurial is a distributed source control management tool.

With it you can save snapshots of your work on documents and go back to these at all times.

Also you can easily collaborate with other people and use Mercurial to easily merge your work.

Someone changes something in text file you also worked on? No problem. If you didn't work on the same line, you can simply let Mercurial do an automatic merge and your work will be joined. (If you worked on the same line you'll naturally have to select how you want to merge these two changes).

It doesn't need a network connection for normal operation, except when you want to push your changes over the internet or pull changes of others from the web, so its commands are very fast. The time to do a commit is barely noticeable which makes atomic commits easy to do.

And if you already know subversion, the switch to Mercurial will be mostly painless.

But its most important strength is not its speed. It is that Mercurial just works. No hassle with complicated setup. No arcane commands. Almost everything I ever wanted to do with it just worked out of the box, and that's a rare and precious feature today.

And to answer a common question:

“Once you have learned git well, what use is hg?” — Ross Bartlett in Why Mercurial?

• Easier usage (with git I shot myself in the foot quite often. Mercurial just works),
• Thoroughly planned features and user interface,
• No need to think much about the tool. There is a reason why hg users tend to talk less about hg: There is no need to talk about it that much,
• Accessing both hg and git repos from one ui via hg-git,
• Versioned tags and the option to use persistent branches which make it easier to track later on, why a commit was added,
• And many great extensions which for example enable much better scaling for huge repositories and distributed teams, along with easy paths to evolve.

I wish you much fun with Mercurial!