Versa competes with sub-compacts such as the Fit, Yaris, Aveo, etc.
Sentra competes with compacts such as the Civic, Corolla, Mazda3, Cobalt, etc.
The Versa is smaller and cheaper than the Sentra (although not by much), and comes with less standard equipment and better gas mileage.
But really, there's a lot of overlap between the two.