What exactly is a "Base Set" when refering to say marvel collector cards?

I found my old cards from when i was a kid, a few of the early 90's sets, and for whatever reason I just really enjoyed looking at them all again. So much so that I was interested in getting back into collecting since I still read the comics. There are a few sets I was looking at and they all offer a base set you can buy but no where says exactly what that is. The card count is less then the complete set, and with its price it is obvsiously not that. Is it just the common cards from the set or is it just a box of random cards from it?

Is it worth it to get a base set if I am going to also just buy a box of the packs aswell? I have tried quite a few sites that sell them and they all just assume I know what comes in a base set.....any info would be very helpful. Thank you