Percent error tells you how far a measurement is from the value it should have, as a percentage of that accepted value: percent error = |experimental - theoretical| / |theoretical| × 100. Measuring gravity as 9.6 m/s² when the accepted value is 9.81 gives |9.6 - 9.81| / 9.81 = 2.14%. The signed error keeps the direction, so a negative value means you measured low.
It depends on the experiment. Under 5% is usually considered good in school labs, and under 1% is excellent, but instruments and methods vary.
Use percent error when one value is the accepted "true" value. Use percent change or percent difference when you are comparing two values with no reference.
The error is divided by the theoretical value, and dividing by zero is undefined.
See also: percent change calculator · percentage calculator · standard deviation calculator