If the object travels in a straight line, no matter it goes forward or backward. This kind of motion is considered to be linear motion. Linear motion has a unique property that makes problems easy to solve. As there is only 2 direction that the object can move. We distinguish the direction by positive and negative sign. Thus, Linear Motion does not involve complicated vectors in motion path.
Because of its convenience in calculation, problems consider the motion on a plane (non-linear) will always resolve all forces into 2 components (2 parametric equations), with one perpendicular to another. This will simplify the questions and we can solve the problems without having to take more care about the vector direction in the calculation process.