From the Document: "Although voting in person at a polling place on Election Day is the most common voting practice, states and localities also offer some or all of their voters opportunities to vote without going to the polls on Election Day. Sending voters ballots by mail ('mail voting') and designating a pre-Election Day period when voters can receive and cast a ballot in person ('early voting') are two of the most common options. Some states and localities use mail voting as their primary voting method, automatically mailing ballots to all registered voters and offering limited in-person voting options ('all-mail elections')."
CRS In Focus, IF11477
Congressional Research Service: https://crsreports.congress.gov/