Varnish, and yes you put it on your paintings. Always. I use it no matter what kind of paint I’ve been working in. It’s used to protect the paints, give it a ‘finished’ look at the end, it seals it to a certain degree from the elements, and makes the colors POP. Seriously, the second you varnish a painting the colors brighten and intensify as you do it, it’s pretty cool to watch. Overall it’s kind of like when people buy iPod cases. Why do they do it? Sometimes it can add protection and a specific look. Same reason you varnish or seal a wooden deck, seals out water, makes the wood last longer and makes it looks good. You should always use the correct type of varnish for your piece depending on what you used in it. It’s not that difficult, the can tells you all you need to know like what you can use it on and when you can when you buy it. And always, and I mean always wait 12-24+ hours to varnish. Sometimes, you can get away with sooner, but that’s something you figure out with experience and getting to know your paints, but it’s risky and very often you can end up destroying your work because you didn’t wait a day. My last painting was my final for an art class, I varnished it after about six hours, cause it seemed dry and the class was in a few hours. And all the blacks bled out over the painting. I had to work like you wouldn’t believe to finish it. I almost cried.