An expression is simply values (variable or constant) and operators. For example:
y = x y = x + y y = -x - y y = (5 + (3 - 2x)) / z
Please take time to look at the operator and casting sections, as many bug reports have been generated by a misunderstanding.
Like C, but unlike Pascal, variables of different types can be mixed freely in an expression. In this case, the promotion rules listed under "operators" are in effect.