Facebook Like is a feature to give positive feedback on Facebook. It's an easy way to let someone know that you enjoy something, without leaving a comment. You can like contents (links, photos, videos, comments) on your friends Wall or Pages through social plugins.
The Facebook Like Button
If you click on the Facebook Like Button, a story will appear on your Wall (and may also appear in News Feed) that you liked a content.
Facebook Pages (for example: Fan Pages) may post updates to your News Feed if you Like them.
You have the control to unlike something you liked before or control who (everybody / a group) can see your likes on your profile.
There is a counter at every Liked content, so you can see how many peoples clicked on it to give positive feedback. Also the Facebook Like plugin has a special format, for example: "Friend1", "Friend2", "Friend3" and 954 others like this, where you can see your friends name emphasized and an aggregated number of people you don't know. The higher number the page has, the more popular the content is.