Facebook allows you to "like" a page, post, picture, or almost anything else. When you like a page, a notice gets posted to your wall. This applies to both Facebook and external pages. Facebook also gives external page authors a widget to add to their page that makes liking the page easy. When you click on that widget, Facebook records that fact, puts a notice on your wall and increments the 'like' counter for that page. Note that this requires you to be logged into Facebook.

In the last week, I have seen several posts by friends 'liking' a particular page. When I checked with those friends, they did not explicitly like that page. They had received the notice from someone else and went to investigate. When my friends clicked on something on the page (e.g., video 'play' button), the page signaled that event to Facebook as a 'like'.

To my knowledge, at this time there is no way to prevent malicious 'likes'. It does mean that you should regularly check you Facebook wall for unintended 'likes' and delete them. Facebook also allows you to report those as spam.