Earth’s gravity isn't uniform, being affected everyday by such factors like winds, currents and tides. The map is actually called a geoid, the result of two years of orbital surveys by the European Space Agency (ESA) Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite (click the link for a animated GIF).
Gravity is strongest in yellow areas; it is weakest in blue ones.
Read more here and go here for the GOCE/ESA website
