enter is used to insert elements into the DOM; if there is data in the data-set which isn't represented in the DOM, then new elements will be inserted which have that data. It doesn't have to do with animations really, though you can animate those newly inserted elements if you wish.
I understand how enter works, but my point is that the naming is pretty poor. If I haven't been around d3 for a while, it becomes very easy to forget how it works because of exactly this. Its api isn't intuitive, which often indicates either bad naming or design - in this case, I would pin it largely on the naming, and the api design not facilitating understanding as easily.
