Well, here's what I mean. On a mag trainer, if you pedal twice as fast, you are working exactly twice as hard. However, in real world conditions, if you are going twice as fast, it is more like 2.3 times as hard, mostly due to the fact that you are spending energy punching a hole through the air. Could be a lot more if you're really going very fast. Wind, fluid, and centrifugal trainers all mimic this effect.
Rollers are a completely different scene than wind trainers.
Three metal or composite drums are arrayed on a metal frame. You put your rear wheel between the two drums in the rear, and your front wheel is touching the front roller. Get up and running, and off you go. Rollers are reknowned for teaching a smooth pedaling style. Rollering is a real skill, and for many bike racers, this is their primary form of indoor training.