Land creatures are both lands and creatures.
They are played like lands but suffer from summoning sickness and have power and toughness like creatures.
Currently only one land creature exists, which is Dryad Arbor from Future Sight. As a Forest it has “: Add to your mana pool” but is technically a vanilla creature.
There are however a number of lands which can become creatures via an activated ability. Those lands are commonly referred to as manlands.