The earth's temperature is hotter at the equator and colder at the poles because of the difference in the angle of the sun's rays. Since the earth is round, the sun's rays hit different areas at different angles; the higher the latitude the more slanted are the sun's rays. In tropical or lower latitude areas the sun stays more or less overhead throughout the year. Since direct rays provide more heat than rays at an angle, the tropics receive the most heat and have the warmest average temperatures.
..hope it helps though
