Health savings accounts and flexible spending accounts offer special tax benefits to encourage you to plan for future medical expenses. However, don't get carried away: You can only contribute to either an HSA or an FSA unless your employer offers a special purpose FSA, in which case you are permitted to use both at the same time.
With a general FSA, you can use the money in the account for any qualifying medical expenses. However, for a limited-purpose FSA, you can only spend dollars in the account on dental, vision and preventative care until you hit your deductible for the year. Once you hit the deductible, you can then use the FSA funds for any qualifying medical expense.