What's the difference between a vector and a scalar?

A scalar is a single quantity giving an "amount" of something - it only has a magnitude. For example, temperature or energy are represented by a single value. On the other hand, a vector has both magnitude and direction. It can be viewed as an arrow which has a length corresponding to the size of the value, and points in a particular direction. For example, displacement and velocity can be represented by arrows. In physics, vectors are very useful in resolving forces. Often, a force acts in one direction but you may want to know the amount of that force which acts in other directions. By drawing a triangle, with the force "arrow" as the hypotenuse, you can decouple the force in the axes that you are interested in (see diagram).