Strait line motion is the most basic form of motion. It is strait ling with no force involved thus no acceleration and a constant velocity. Circular motion is motion around a central point it results from a force being applied to an object at a right angle to the objects motion such as gravity keeping a satellite in orbit is a good example.
Relativity theory shows that all motion is relative since it can only be measured relative to a given observer. These measurements are also reversible such that if I see someone moving a given direction at a given speed, he sees me going the opposite direction at the same speed.