If you look at examples in a physics book of scalars, you'll see a common property that unites them to the category scalar.
If you look at the examples of vectors, they will have a common property that unites them to the category vector.
Scalars use only a one dimensional coordinate system, whereas vectors use a coordinate system with more than one dimension. This is the differing principle: one dimension (scalar) and more than one dimension considered (vector).
Consider the Cartesian coordinate system. There is the x, y and z axis. Single one out and it's only one dimension. Add two together and you get two dimensions. Add all three and you get three dimensions.
The word 'scale' sounds like scalar and is in the scalar category. A bathroom scale plate compresses a spring along one axis, which is one dimension, the 'y' axis and dimension, which is soley up and down. This spring causes the needle to move on the scale demonstrating the magnitude along the y axis, which is the linear displacement the scale plate moved from the point it was before you stood on it to after you stood on it.
When you play chess the Rook needs only one dimension to move during a turn, which is the x or y dimension. The Rook may move in either the x or y direction, but may not move in both on one turn. So, when the Rook does move, it relies on one dimension only. The Knight must rely upon two dimensions to move. It moves one or two spaces along one dimension then two or one along a separate dimension.
A thermometer soley measures magnitude along a linear axis. It is a scalar only because it considers one dimension of magnitude, which occurs along the number line(one dimension) where you read the temperature.