So I want to buy a head, but am unsure of everything.

I'm in the market, or will be soon, for a head and cab. I wanted to know about them, because I have no idea about amps and such. One thing I was particularly unsure about was the term "Slave Output". Keep in mind that I'm looking on ebay and pawn shops for used gear, because I'm not a millionaire, and can't afford new stuff. This makes descriptions harder, and even on company web sites it is unclear. Another thing I was curious about were the different channels and is it bad to have only one channel? What do you guys know about these boutique (might be the wrong word) manufacturers like Orange, Mesa Boogie, or Hughes and Kettner compared to more general brands like Marshall, Vox, or Peavey?

What about the cab? Do I need the Ohms of the head to match the cab? What if the head has different outputs like 50w and 100w settings?

Any input will help.

THANKS! :)