At my middle school, our textbooks would consider x+7=12 to be "pre-Algebra" because it is easily and quickly calculated without any rearranging of the equation, does not require advanced calculation. Many kids could do it in their heads if they weren't worried about that elusive x in there.
Algebra would be problems like ColinsMum has posted. We also include fractions in there where the variable is the denominator or where there is a variable on both sides of the equation. We also consider graphing equations and graphing inequalities to be Algebra.
