It is perfectly normal to take the SATs more than once. Starting in March 2009, you can benefit from a new policy that lets you pick which one of your scores you want colleges to see. Make sure you select that option if that's what you want to do.
English Language Arts - Grammar and Usage section/Critical Reading
Writing Assignment - spontaneous composition to assess your written expression in terms of creativity, clarity, relevancy, effectiveness and use of detail.
Mathematics - Problem-solving abilities including multi-step problems involving estimation, computations with fractions, decimals, percents and whole numbers, rules of divisibility, simple probability, rate, average, ratio, time, money, area of shaded regions, perimeter, counting, visual and numerical pattern recognition and three dimensional figures.