You can set the format that a deck is designed for. This is useful for finding specific decks using the format filter menu.
To set the format for a deck, go to the Details page for that deck and use the drop down menu marked "Format".
You can select from the following formats:
Standard
Modern
Legacy
Vintage
Commander
Planar
Vanguard
Scheme
Conspiracy
Tiny Leaders
You can check whether your deck is legal in a particular format by clicking the "Show legal formats" link underneath the Format menu. If your deck is not legal in a format, the format name will be greyed out. Hover over that format name to see the issues preventing your deck from being legal.