Backtrack is a Linux live CD. So you can just drop it in your drive and it'll work, without having to install it to a hard drive. As timmedin said there are many many different flavours of Linux, what's special about backtrack is it comes with a mass of pentesting tools installed, so you're good to go straight away.
Because BT is tailored toward the pen tester, it's actually quite small (1CD), where other distributions of linux can have many CDs or DVDs, so it might be a good choice if you're limited for bandwidth, however if you're completely new to linux you might want to take a look at ubuntu which has a massive community for support. (and there's also a Live CD version)
CCNA, 100m Swimming cert.