With a prepaid tuition plan, you prepay all or part of a child's future tuition by investing in units or contracts (depending on how the particular plan is structured), and you're guaranteed a minimum rate of return. However, you aren't necessarily entitled to any extra money that the plan may earn.
With a college savings plan, you contribute to an individual investment account to pay for a child's future education. Your money is invested in a particular investment portfolio at the time you join the plan, and you take your chances on what your rate of return will be--there are no guarantees. If your portfolio performs well, you reap the benefits. If it doesn't, you suffer the losses.