channel = distinct audio signal. eg, left channel and right channel. or left-front right-front ect... or center or subwoofer (for various applications). basically a channel is an actual amplifier. a 4 channel amp has 4 amplifiers in one case.
channels isn't the number of speakers. for instance, a two channel amp can be combined to work on one speaker (bridging). and multiple speakers can be run off a single channel. (in home line arrays, sometimes 60 speakers are run from a single amplifier channel!)
as for "ohms", basically it stems from power. high power is low voltage and high current, or high voltage and low current. 8ohm home speakers are more for high voltage with low current. but in a car, the battery is made for low voltage, high current. thus in a car, it was decided that 4ohm speakers would be better.
for aftermarket speakers, the choice of speaker impedance (ohms) is based upon the amplifier used. keep in mind that multiple speakers can be combined in various combos to get different impednaces. this topic is important as there is typically a choice between 4ohm(1ohm) and 2ohm. choosing the correct one will allow you to get full power from the amplifer, while the other will allow only half power.