Right, easy stuff first..... open-limping is when you are the first in the pot and you don't raise (open-raise), limping, by itself can sometimes mean this, too, but is generally referring to when one or more players have already limped-in before the action gets to you.
Basing your limping/raising when you are first to act upon the strength of your starting hand is generally pretty bad since it gives away the strength of your hand pre-flop if you always limp weak hands and raise strong ones. Plus, if you are EP and limp something like A-rag suited, you are giving people left to act good odds
to flat. This is going to make it awkward for you to continue with the hand almost always.
If you flop the Ace, it's likely you're going to be out-kicked. If you flop your weak kicker, there are likely going to be overcards on the flop which may or may not put later players already ahead of you. Lastly, if you flop nothing, which you frequently will, players in LP who called your limp can represent to have hit just about any flop whther they actually did or not since you let them see the flop for cheap.
There are all kinds of variations on this, of course, and much more could be written, but it's generally standard to say that if you are the first to enter the pot, and your hand isn't strong enough to raise, you should probably just fold.
Limping after others have already limped, though, can be correct at times, given implied odds. Even then, though, you may want to consider an isolation raise pre-flop since you'll often either win the pot right there, or you'll play the rest of the hand in position v. the limp-caller.