The R series ThinkPads are consider budget line of the ThinkPad laptops. The RXX series lacks some of "normal" R series features like stereo speakers, touchpad, hot-swappable CD/DVD drive, docking connector and some other things. They also usualy feature slower CPU like Intel Celeron (but in newer models also slower Intel Core processors). ACPI functions in modern RXX-e models are just as well supported in Linux as they are for the regular RXX models.
Starting from R61, the R series ThinkPads feature the same chasis (including RollCage) as the T series making them virtually undistinguishable from each other.