A Python expression like a mathematical expression has a value: we say that it "returns" a value.
Example of an expression: 3 + 4
A statement is a Python instruction.  It has a side effect, i.e. it does something, but it does not return anything, i.e. it doesn't have a value.
Example of an statement: x = 3 + 4
An statement can contain an expression, but an expression cannot contain a statement.
