A general proxy lets the holder vote as they see fit; a directed proxy tells the holder how to vote on specific questions. Proxies also count toward quorum, which is often why boards collect them.
Governing documents and state law set the rules: the required form, whether it must be signed and dated, when it must be received, how long it stays valid, and how an owner revokes it. Some states limit or prohibit proxies for certain votes, such as board elections, in favor of absentee or electronic ballots.
Keep every proxy with the meeting records, and have someone other than a candidate verify them before the vote.