Order of operations
Brackets, then exponents, then multiply and divide left to right, then add and subtract left to right.
A worked example
Work out 2 + 3 x 4^2. Exponent first: 4^2 = 16. Then multiply: 3 x 16 = 48. Then add: 2 + 48 = 50.
The mistake people actually make
Multiplication does not always come before division. Work left to right when only those two are left.