Also how reliable is the site posted by Crashdamage?
For the 3 or 4 carriers I checked, they're accurate, even to future expansion plans. As far as I know, AT&T is using a single band all over the country, but they're listing the ones AT&T plans to expand to, so you'll know whether the phone will continue to work in the future.
I'm new to the technical world, so what does unlocked bootloader mean?
It means you can put in a custom ROM, an custom kernel, a custom recovery - you have control over your phone, the carrier doesn't. A locked bootloader is like a desktop that can only use WD hard drives and Radeon graphics cards. Or a car that can only use BP gas. You're getting half a phone. (I'm not new to the technical world, I've been in it for ... well, I took my first radio apart in about 1949 [couldn't get it back together without help, but it was a start], and I still bought an AT&T phone with a locked bootloader. AT&T wouldn't recognize it now, except for the logo on the case, but I still can't do everything I want to it.)