The accepted answer is great, I just wanted to point out that this feature is why some people make maxing out HSA contributions a higher priority than IRA's. With an HSA you can get pre-tax contributions, tax-free growth, and tax-free disbursement if used on qualified medical expenses. The longer you wait to get reimbursed for qualified expenses the more tax-free growth you can get.
Even if you don't want to sit on receipts for years, contributing to an HSA and not using it immediately is still useful in retirement. Once you hit 65 you can use the HSA to pay for Medicare parts A, B, D and Medicare HMO premiums. Also at 65 you can take penalty-free distributions for any reason, but you would pay income tax on those distributions (like a traditional IRA). Over-funding an HSA isn't much of a concern.
These rules could change in the future, but currently the HSA offers fantastic tax advantages. One important caveat, is that not all HSA's have good investment options, so evaluate your plans investment options before making HSA a priority.