Project Constants
Constants are name-value pairs that are visible across the Project. Using a constant allows you to define the value once, and then refer to it from the various Project configurations, rather than having to redefine the value everywhere it is needed. For example, you might define a standard currency to be used globally throughout the system.
Project Variables
Variables function as placeholders, having values that are determined when you create a specific Deployment Profile. They can be either literals or Environmental constants.
Constants and variables are discussed further in Defining Constants and Variables.