Forest is a land type. Dryad is a creature type.
Dryad Arbor is played as a land. It doesn't use the stack, it's not a spell, it can't be responded to, it has no mana cost, and it counts as your land play for the turn.
If a Dryad Arbor gains flash, or you have the ability to play Dryad Arbor as though it had flash (due to Teferi, Mage of Zhalfir or Scout's Warning, for example), you can ignore the normal timing rules for playing a land, but not any other restrictions. You can't play Dryad Arbor during another player's turn, and you can't play Dryad Arbor if it's your turn and you've already played a land.
If Dryad Arbor is changed into another basic land type (such as by Sea's Claim), it continues to be a creature and a Dryad.
Although originally printed with a characteristic-defining ability that defined its color, this card now has a color indicator. This color indicator can't be affected by text-changing effects (such as the one created by Crystal Spray), although color-changing effects can still overwrite it.