"+1 is as simple on the rest of the web as it is on Google search. With a single click you can recommend that raincoat, news article or favorite sci-fi movie to friends, contacts and the rest of the world. The next time your connections search, they could see your +1's directly in their search results, helping them find your recommendations when they're most useful," explains Google.
The button can be customized: you can hide the number of +1's, choose a different button size or explicitly load the button. "By placing the tag at the bottom of the document, just before the body close tag, you may improve the loading speed of the page," suggests Google.
Right now, very few websites include Google's +1 button, so installing a browser extension could be a better option. Unfortunately, Google hasn't released yet a +1 extension.