There's a good chance you've seen Lulz used on the internet, typically on social media, message boards, or forums.
Acronyms are abbreviations of words used to cut down on the time it takes to type. Acronyms are often typed and easier to understand when typed in capital letters.
There's no shame in seeing an acronym you don't recognize, and it's happened to everybody (or it eventually will).
The short answer is that Lulz is similar to LOL (Laughing Out Loud).
Lulz is LOL, except that it is often at someone else's, or one's own expense, as a prank. For example, it might also be used when someone asks why you posted something.
"I did it for the lulz."
To some people, using Lulz seems sillier than lol, hence more fun to say in those situations. We agree.
So, while you might use lol to say you're laughing out loud, you might use lulz to say your laughing at someone or something.
You might also see other LOL variations, including LOLz and LOLs, geared more towards Lulz than LOL.
While any gender or age might use lulz, the older one gets, the less likely they are to use it.