Rider-Waite uses the Eden story for the Lovers. In the picture, they are standing between the Tree of Life (maybe the burning bush?) and the Tree of Knowledge, and there is that snake signaling choices. As they are naked and not wearing fig leaves, we don't know if they've eaten of the fruit yet or not, or if they've been returned to Eden for that matter.
The Angel above takes the place of "cupid" here signaling their spiritual marriage and the spiritual communication that is possible between them, as they are two halves of a whole (Eve came from Adam's rib, so she and he are one in the same, opposite sides of the same coin). The Angel is, if you will, emblematic of the telepathy and understanding between couples made possible by love.
He may also symbolizes "banishment" from Eden. But Waite believes that there is a way back to Eden as well. People look at this card and see only Adam and Eve being sent out of Eden, but the card could just as well be showing their return to Eden with the Angel welcoming them back. Back to perfect love.
The serpent represents the choices we make in life and love, and the consequences of such choices. It also represents, like the Angel, a connection between Adam and Eve. As if one eats of the fruit and the other does not, they will be separated.
Personally, I don't much like this image for the Lovers. Earlier decks usually had a man choosing between two women with cupid overhead. I tend to like those better. They match, IMHO, the original name of the card better--that name being just "Love."