Re-raising??

I'm fairly new to tournamnet poker - both live and online. I play online in mostly $1/$2 NL Holdem games and slightly higher and lower limits. I checked my online stats and saw that I had never re-raised. I've noticed this before on other sites - I rarely if ever, re-raise. I don't think I've really seen a section in a book on when to re-raise - nor do I remember reading even a paragraph on this subject!
Any general advice on when to re-raise?? Certainly on the river if you have the nuts and somebody raises - obviously a re-raise is in order. Is the online definiton of 'reraise' a raise of my raise - or just a raise of any player's raise. I think of it as someone re-raising my raise! You can see the problem - I don't even know what a re-raise is - no wonder I never do it!! Any advice on ra-raises?? Not just on the river - but anytime? Thank you!!