In deck construction, a girder is a board or a composite of more than one board that serves as a supporting beam for the deck joists, which meet it perpendicularly.
When building a deck that will be adjacent to a house, joists are often attached at the house end using a header board. The joists will be "hung" from the header board using joist hangers, keeping the tops of the joists at the same level as the header board. At the opposite end, support is provided by the deck girder. Rather than being hung from the deck girder, the joists commonly rest on top of this beam. The deck girder, in turn, rests on posts for support.
When building a deck that will not be adjacent to a house, there will be a deck girder at both ends of the deck for support. Indeed, even when the deck is to be adjacent to the house, an alternative construction method is to use a second post-and-girder system rather than tying the deck directly to the house via a header board.