a "cab" is short for "speaker cabinate", you need a speaker to make sound, a "combo" is an amp and a speaker in one box, a "head" is a seperate amp. the bigger the speakers in the "speaker cabinate" the lower the sound frequencys, the more bass. with smaller speakers you get more treble and less bass, this is the rough rule, there are exeptions. there are lots of diffrent types of speaker cabinates, from little ones with single speakers, to massive ones with manny speakers. if you have a "combo", often by adding more speakers you can get more volume. the amp needs a certain ammount of resistance, the speakers give resisitance, the more speakers the more resistance, the more volume. some speakers have a lot of reisitance some only have a little, some amps can power a lot of speakers, some can only power one. now i hope this is a rougth overview of cabinates, now its starts to get complicated, you gets onto "ohms" the measure of resistance, do a search and you will find loads of info. if anything i have said is incorrect feel free to correct me.