Most cabs are either 8 Ohms or 4 Ohms. The lower the resistance, the more power will be extracted from your amplifier, so if you plug your rig through an 8-Ohm cab, and then plug it through an otherwise-identical 4-Ohm cab, the 4-Ohm can will sound much louder. NOT twice as loud, but a pretty big step up.
Life gets exciting when you start combining cabs in pairs. For reasons that you can look up on t'Internet, if you pair two 8-Ohm cabs then their combined resistance drops to 4 Ohms, and if you pair two 4-Ohm cabs then their combined resistance drops to about 2.7 Ohms.
There are plenty of amps that can handle resistance as low as 2 Ohms, but there are plenty more that can't. The easiest way to tell the difference is to look for the tell-tale white smoke coming out of the back of the amp. This is not a design feature.
As a general rule it's best not to get hung up on wattage figures. They are horribly misleading, easily misunderstood, frequently misrepresented, and tell you surprisingly little about how loud a cab will go. Trust your ears. Go and see half a dozen bands playing the sort of gigs that you expect to play, check out what the bassist is using and how he/she sounds, go and have a chat between sets. Find a sound you like and see whether you can buy something to match.
You'll sell it within the year as you learn more, but that's the price of spending time on Basschat.