ABC is a format for writing tunes in a portable and easy way. It was invented by Chris Walshaw, and is used for sending tunes via e-mail, for putting tune books on the net, and for typesetting tunes. Over the years it has become the de facto standard file format for traditional music. There are a number of different programs available for handling ABC files on various platforms, which can convert ABC files to standard notation, play ABC tunes through MIDI, search for tunes, etc.
More information about the ABC format is available at The Official ABC Home Page. The page "learn abc" is particularly useful for ABC beginners. On the ABC Home Page you can also find links to programs and apps for printing sheet music from ABC files, etc.
For Windows systems, I made the program Abcmus for playing tunes via MIDI and handling tune books.
I can also recommend the site and app TunePal for finding ABC tunes.
Also check out my ABC links.