(Dryad Arbor isn't a spell, it's affected by summoning sickness, and it has ": Add to your mana pool.")
5/1/2007 Forest is a land type. Dryad is a creature type.
5/1/2007 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.
5/1/2007 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.
9/22/2011 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.
6/7/2013 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.