A field or pill in blue is Discrete. A field or pill in green is Continuous. Dimensions and Measures can be either Continuous or Discrete. 

A Discrete field displays each distinct value. Any data types can be expressed in a discrete way. In the View, a Discrete field placed in Rows or Columns is represented with headers. Here is an example of a Discrete Dimension and a Discrete Measure, as you can see both have clickable and distinct headers:

A Continuous field represents values from an infinite set. Only numbers and dates can be continuous. In the View, Continuous fields are represented on an axis. Here is an example of a Continuous Dimension and a Continuous Measure, as you can see, both are displayed on an axis:

It is crucial to understand these differences. It may sound a bit abstract, but it'll help you when you start building visualizations. Talking about visualizations, there is a bar that you constantly see when using Tableau: the toolbar. Let's see the different options available with a single click.