Both accounts provide a pre-tax contribution option through payroll to be used towards your healthcare claims, copays, dental, vision, and prescriptions.
An HSA is only available with a High Deductible Health Plan; this account has an annual maximum that can be contributed, but it does not have a lifetime maximum and all unused funds roll over each year.
An FSA is available for medical expenses and dependent care expenses. The annual limits are set by your company and the funds are only available for a 12-month period. Any unused funds are forfeited at the end of your 12-month period.