Earned value management (EVM) is an analytical technique used to control costs. By calculating the earned value and comparing it to other amounts, the project manager can determine if the project is on-course to complete within budget. EVM compares measures of the key constraints of time (schedule), cost (resources), and scope.
See section 22.214.171.124 in the PMBOK 5th edition. The PMBOK also recommends referring to the Practice Standard for Earned Value Management -- Second Edition.