Possible Duplicates:
For home projects, can Mercurial or Git (or other DVCS) provide more advantages over Subversion?
What are the relative strengths and weaknesses of Git, Mercurial, and Bazaar?
What are some of the differences between these source control system? Which one is the best for a small 2 people project?
To start with, there's the language they're written in. My experiences with Git and Mercurial have been very similar, but I know that if I want to tweak Mercurial, I can do it, because it's written in Python. Git is at least somewhat in C, which I'm not as familiar with.
Git and Mercurial are what's called distributed. Every copy is created equal, and they can push and pull (using that terminology) changes from each other on an ad-hoc basis. Subversion, on the other hand, consists of a single central repository, and each working copy is a slave to that central server, pushing and pulling (committing and updating, in this case) changes from it and it alone.
Installing Git or Mercurial for a couple of people consists of getting SSH access to the same server and installing a couple of packages. Whereas for SVN, as far as I know you need to configure and run an actual server application under Apache, and then mess with an SSL cert and .htaccess, etc. to secure it.
For all my personal projects, I go with Mercurial or Git. If I were working with a large team, I'd probably go Subversion because you get centralized authentication and hosting. But for two people, I'd pick one of the distributed ones, because then you don't have to mess with centralized authentication and hosting. :-)
SVN is a different from Git and Mercurial, in that it is a single repository that all users have to pull and commit too.
Git and Mercurial have a distributed model. This means that there is a repository on every computer and that their is usually a "Official" repository that people will choice to commit their changes and pull from.
Git and Mercurial are extremely similar. I prefer Mercurial because I found it much easier to use. For a 2 person team I would recommend mercurial, but that is just my opinion. If you are not familiar with version control then you are still gonna have to spend your time learning to use any of the options, but mercurial seemed the easiest for me.
To start a repository all you have to do is open a shell script and cd the the directory you want to have version control and type
hg init
That creates the repostory to add a file every file in the folder to the repository type
hg add .
To commit the changes type
hg commit -m "Descriptions of changes"
To pull the lastest version of the repository from the server type
hg pull
or to push the local changes type
hg push
That is all you need to know for basic version control.
If you need any help please feel free to contact me.
Git and Mercurial are quite similar (but different enough to warrant caution). SVN on the other hand is quite different: the first two are distributed VCSs, so they do not require a central server, while SVN does. In general many projects are moving toward distributed systems.
For your small project, you're probably better off with Git or Mercurial. Which one you choose is essentially a matter of taste, although I prefer Git myself (and am far more familiar with it). You need not set up a server at all: you can push/pull changes through SSH or even email patches to each other (this can be done directly from the VCS but is sort of a hassle). You can set up a central server at any time, and all the changes will be there. You can use e.g. GitHub or Gitorious to host your project (if you're going with Git, I don't know about Mercurial).