Your weight is different at the poles from the equator because at the poles, you are relatively motionless with respect to the earth. At the equator, you are traveling in a circle which moves 25,000 miles in a day; over a thousand miles and hour to the east. That circular path also requires you to accelerate a bit toward the center of the circle (or you'd travel in a straight line). This acceleration due to circular motion is the same as the force you feel on a string when you whirl a weight. Effectively, that acceleration cancels a bit of the acceleration of gravity -- you weigh less at the equator than at the pole of the earth.

There are differences in gravity acceleration due to the shape of the earth and to differences in composition -- but these are much smaller effects.(ie. the weight difference from centripetal acceleration is about a percent or 2, while shape and mountain ranges are about 0.002 percent...)