Lol is a common term on the internet. If you weren't aware, it stands for "Laugh out loud," or as I prefer, "Laughing out loud."
Although it is a mere three letter word, a lot of meaning can be packed behind it, based on how it is written. Let's take a look.
#1: LOL!
All caps, with an exclamation point at the end. This person might be laughing so hard that they're peeing their pants, or their skirt, or whatever they happen to be wearing. Chances are, they probably aren't, but if someone types "LOL!" it's always fun to picture them as doing such. In any case, it's probably safe to assume that they're at least, as the acronym suggests, laughing out loud.
So, the next time you're chatting with someone, and they say"LOL!" you can be rest assured that you've just said something incredibly funny, and if you like, you can picture them peeing in their pants.

#2: Lol!
This difference is fairly subtle, but it's there. While not in all caps, there is still an exclamation point, suggesting that what you've said was funny enough, and the person typing it is most likely smiling to themselves, at least. Sure, you don't get the pants-peeing reaction you'd hoped for, but keep reading, and you'll start to realize how lucky you are to receive a "Lol!"

#3: lol
A plain and simple "lol" with no capitalization, or punctuation of any sort can usually be taken as a quick "Haha, very funny," and is probably the most common usage, sad though it may be.
A person who types "lol" is usually not even smiling, and is probably just filling space in a conversation. "Lol," with no punctuation usually doesn't even mean that anything you just said was in the slightest bit funny, but anything is better than an awkward silence, right?

#4: "Lol..."
Lol, dot, dot, dot.
You've just said something you thought was hilarious, and possibly genius, and your conversational partner replies with "Lol..."
You've probably either just offended them, or made them think to themselves, "Wtf..."
Don't fear, it can easily be repaired with a good old fashioned subject change.
In this case, it is certain that your friend is not even smiling, much less laughing.
But better luck next time!

#5: "Lol."
How many people actually type out "lol" and then punctuate it with a period? By this point, you'd better just consider giving up trying to be funny, because the person who wrote that is probably not in a good mood, and trying to make them laugh, will only succeed in making them angry.
You can picture something that scares you, if it helps, like a vicious clown, or George Bush.
"Lol." is on par with "Lol..."
The biggest difference between the two is that unlike #4, who was likely offended by your statement, but chooses to stay calm and quiet about it, #5 might not always stay so quiet about their feelings, and e-drama will likely ensue, so unless you're a master troll, let this response be the end of your conversation for the day.

Now, perhaps I haven't picked out every single variation of lol, but they're easy enough to decipher if you take a moment to look at them under a microscope and judge them accordingly.
And surely, there are exceptions within the degrees, but you'll just have to decide for yourself based on the nature of the conversation.

About Author / Additional Info:
Harmony Schreiner is the author of "Canta Per Me," and "Call," the latter of which has yet to be released. If you found this article informative, or amusing, you can learn more by visiting her website.