Ritardando just means to slow down. In the classical orchestras, jazz groups, and concert bands I've played in, it's really up to the conductor to interpret how much and how gradually to slow down. As for Rallentando, I really have no idea. I've been playing in ensembles for 12 years now and have never encountered it. I've heard it described as a more abrupt slowing compared to Ritardando, but a quick internet search suggests that it means different things to different people. When playing without a conductor, I think it's up to you to decide how to execute a Rit.
I see both used in the banjo sheet music of the period and never did figure out which was which (and simply use them interchangably).
I like the explanation in Jody's link. Ritardando = slowing down your car using the brakes. Rallentando = taking the car out of gear and letting it reduce speed by coasting.