A voting is a part of a topic. It's a place where users can cast their votes and see voting results.
To cast a vote, go to a voting's page, then select your option (left menu) and click the vote button. If you're rating an item on a 1-10 stars scale, simply click a star. When your vote is successfully saved, the graph animates, showing the new voting results (including your vote).
To change your vote, simply select other option and click the vote button again.
Some votings allow you to select more than one option in a single vote. In such cases it is stated right above the first option. Example: (to select: 3 max.).
Every registered user has a voting panel in which he can see all of his votes. More info at Help: My account.
Every user can hide his votes from being viewed (on his public profile) by other users. This setting can be changed at My Account > Privacy.