Like button #1 At the top of (nearly) every thread there is now a "Facebook like" button.
This button can be found all over the web. If you click it (and then click "yes") a link to whatever website you were on will be posted to your Facebook wall. No information from the website is transferred to Facebook, just a link to the page you were on. A button with this functionality (and one that does the same for Twitter) has been in the Viewport bar for a number of months now.
Like button #2 Recently there has also been discussion of having a "like post" button.
This is a common feature in many forums. If you click it, the count next to the post goes up. "3 users like this post". Sometimes it will say "Jenn, Worley and Vools like this post". It is entirely internal to the forum and has nothing to do with Facebook. MNHQ said that they might trial this on a limited number of topics.
If you cannot tell the difference between these two then (IMO) you are not qualified to have an opinion on either.