Petya is the latest ransomware hitting millions of computers around the world. Most infected computers are in the Ukraine, where "patient zero" is believed to be. From there it branched out to Russia, Poland, Italy and Germany. It takes advantage of the same flaws in Windows 10 that WannaCry did. Fortunately, it hasn't really hit the U.S. yet, but we'll see more infections as time goes on. Our CIA intelligence service discovered it and didn't say anything because it could use it to spy on others.
WannaCry is ransomware that can lock up your data unless you pay the hacker who created it. WannaKiwi, however, finds the crypto key in your PCs RAM to undo the damage. It only seems to work about a third of the time, however. That's why Leo says to make sure you don't get it by altering your behavior, and by making sure you have current backups of your data should it happen. One thing you should never do is pay up, because you don't know if you'll get your data back, or if there's something even worse getting installed.
Last weekend, the WannaCry Ransomware bit several hundred thousand computer systems, including sixteen hospitals in the UK. The ransomware infected the systems and encrypted all data. The reason this one was really bad is that it was a "worm," or a "network aware virus" that would spread out over the local area network to find other computers to infect, and bring the whole establishment to its knees.