Approval voting is a really simple type of voting, and unlike most voting methods supported by OpaVote, does not use a ranked ballot. You can see an example ballot on this page that could be used for approval voting.
With approval voting, you have the option to approve of any number of candidates. Candidates you approve get a check mark, and candidates you don't approve of don't get a check mark. The candidate with the most approvals is the winner.
Although very easy to implement and count votes, one tricky aspect of approval voting is how to determine which candidates to approve. Clearly, a voter should approve their favorite and not approve their least favorite, but whether to approve other candidates is a tricky decision. For example, approving your second favorite candidate could cause your second favorite candidate to beat your favorite candidate.
You can see an example of approval results generated by OpaVote.
No governments currently use approval voting for elections.