Ransomware is a type of malicious PC shit that makes your computer get locked down, and some flavors of this even encrypt your drive. Pretty much, it's a really forward scam that forces the less tech-savvy to pay just to get access to their computer and data.
Super legit, and even the most tech savvy can't get around it sometimes. Certain kinds of ransomware will essentially destroy your ability to use that computer without a complete reformat.
Fear not, though! Most varieties of ransomware can be removed simply by booting into safe mode (just google "safe mode" and your Operating System[XP, windows 7]) and running a program like Malwarebytes. Alternatively, if you have access to the internet on a non-infected machine, just google "how to get rid of <insert name of the site trying to extort money> ransomware".
WHATEVER YOU DO, THOUGH, DO NOT LET THAT COMPUTER ON YOUR NETWORK IF THERE ARE OTHER COMPUTERS ON THAT SAME NETWORK. Ransomware spreads crazy fast, so consider yourself warned.
Simple, these are pretty much exclusive to scam porn sites and email attachments. Just exercise some common sense and don't open emails all willy-nilly, dummy. If you aren't tech savvy, then leave the security notifications on in Windows 7 and up, and make sure to keep your antivirus/antimalware programs up-to-date.
(There, happy MrFixIt?)