Voltage is that thing which you use when dealing with cordless jigsaws, 18V is going to be sufficient for most jobs at the cordless end.
AMP stands for amplifier and is related to the general energy of a corded saw. The higher a saw's AMP, the more power it can put behind every stroke. These numbers typically range from 4-7. 5 usually are enough for all but the hardest materials you can expect to cut.