Dimension is a measure of a physical quantity without numerical values. It is divided into two categories namely fundamental or basic dimensions and derived dimensions. There are seven primary dimensions from which derive secondary dimensions.

Examples of dimension are mass, length, temperature, electric current etc.
A unit is a way of assigning a number to dimension. Units have different systems of notations such as SI system, M.K.S system, F.P.S system etc. For example, time is a dimension that is measured in days, hours , minutes , seconds , milliseconds  etc.
Examples of units are kilograms, meters, centigrade, amperes etc.