Variables, Expressions

Variable- a letter that represents an unknown number.

E.G. x, a, n

Expressions- any single number, variable, or combination of operations involving numbers and variables.

E.G. 5, n, 8+, x+1, 2y-7

for more help check out

http://go.hrw.com/resources/go_mt/alg1/alg1refresher.pdf

Or you can check out this video