What exactly do scripting languages do and which one should I learn: Ruby, Perl or Python?

I already know PHP, but I’m still not sure what the functionality of these 3 languages is… I can probably make a good guess, but I’d still like to know what they’re used for. Also, which is the most useful, generally speaking? I’m wondering which one to learn first. I learn extremely fast, so that’s not an issue.
Thanks a lot