If you repeat the words to a song that is copyrighted (and almost all are, save a few old ones in the public domain), and you do not first obtain permission, then you've probably infringed on the copyright owner's "exclusive" rights. What happens after that depends on what the owner wants to happen. Because YouTube gets more "eyeballs" than almost anything else, and it does not want to be an accomplice, as soon as someone claims that you've infringed, they'll take action (without actually investigating whether the claimant has legal grounds, I might add).
Now, a lot of song copyright owners have already worked with YT to establish what they want done - most common is you'll simply get a notice and YT will stick more ads into the video, and any revenue generated goes to the copyright holder, not you (assuming you're eligible for monetization). A few do say they want the video removed, and that's when you get a strike against you.
I post covers but I go through the steps to obtain a streaming (mechanical) license for the audio. If that's not possible, then I don't post the video. Video licenses are tedious and expensive, and you can assume if you can get the audio license, YT probably already has the video side covered, especially if there are already multiple copies posted. The corollary here is that if you don't see any other covers, it's probably being aggressively protected, so go look for something else to cover, or at least post.
Places like SoundCloud and SoundClick generally aren't in the monetization business, so you can probably get away with more there, but it doesn't mean that you aren't in violation, and the best course of protection is to pony up the money for a streaming license. (EasySongLicensing.com is one site I've used - the "handling/processing" fee is the most expensive part most of the time.)
This is all very simplistic, and IANAL, so if you really want to know what to do, talk to a lawyer, read lots of very confusing books (this stuff changes over time), or consider writing your own songs
.