Using the site
If you just want to browse around and see what's here, this all probably doesn't need much introduction, it's just another website. It stores tunes, and can make up printable lists of them, which it calls "documents". These can all be viewed here, or downloaded in various formats. There is also the ABC Converter, where you can try out the editing facilities or paste a tune in just to see what it looks like, without having to log in or register (there is, in fact, nothing at all to be gained by registering unless you want to contribute stuff into the database; in which case it's obviously necessary).
The site uses ABC notation - this is a "sol-fa"-type scheme, where you use letters for the notes, and numbers for their lengths, and so on. It's not hard, and works very well (see the More Details help page for more info on this).
There are 2 ways of getting material onto this site - you can either use a pre-existing ABC file, either by pointing the site at a URL for it or by uploading it here (most of the material here has come this way) or by entering it directly using the editing facilities. Whichever way you use, you have full control over which people are allowed to see it, you can keep things private for your own use if you wish to. This may be important with respect to copyright considerations, as well as any personal reasons anybody might have.