Dimension and Measure

When you create a data source, the fields are split between Dimensions and Measures. The Measure is what you want to analyze, and the Dimension is the angle of analysis.

By default, numbers are Measures, and the other data types (Text, Date, Geographical Role) are Dimensions, but that's not always the case. Any data type can be either a Dimension or a Measure.

A Dimension contains qualitative information. It always splits the Marks and is never aggregated. 

A Measure is, by default, aggregated, contains quantitative information, and is almost always numeric.

As you can see, it's hard to give an exact rule to discern Measure and Dimension. It's more a concept to understand and a useful way to arrange the fields. Yes, you could have believed that Dimensions are blue and Measures are green. It is an easy mistake to make when you start using Tableau. 

Curious to know the real difference between the blue and green fields? Everything is explained in the next topic.