I work for a Third Party Administrator that handles for Flexible Spending Accounts (FSAs both Medical and Dependent Care) and Health Spending Accounts, or HSAs.
FSAs involve pretax dollars set aside for the duration of your employer's plan year (usually follows the Health Insurance's plan year) for medical expenses not covered by insurance. As you know, this is considered a "use it or lose it" type of plan. If you don't spend all of your pretax dollars by the deadline, the funds are forfeited to your employer. On the other hand, in the case of a Medical Flexible Spending account (Dependent Care FSAs operate a little different), you are entitled to spend all of your election (what you say you are going to set aside for the plan year) from day one. For example I could make a $1200 MFSA election for my employer's 1/1/09-12/31/09 plan year. In February I have medical expenses totaling $600. I've only put in $200 so far to the plan, but I am able to claim and get reimbursed for the full $600. In March I terminate employment and take a job elsewhere. My employer is out $500 and they can't go after me for those funds or else their plan would be out of compliance with the IRS.
Okay, so that is an FSA. An HSA is similar to an FSA but it has a major key difference: You must be HSA eligible, meaning you must be enrolled in a qualifying High Deductible Health Plan, or HDHP. HDHPs are great if you're healthy and only want insurance for a catastrophic event. If you're not healthy and/or have a large amount of regular medical expenses I wouldn't really sign up for it.
An HSA is kind of similar to an IRA; you can invest the funds and use it as a retirement vehicle. But the main purpose is to invest pretax dollars to be used for medical expenses. So these can be rolled over year to year, as opposed to the "use it or lose it" FSA plan. The downside to an HSA is that your contributions are capped each tax year, right now the limit is $3000 for single coverage and $5950 for family coverage, although you can do a a "catch-up contribution" if you are age 55 or older.
OK tl;dr HSA is different than FSA, google it.