Hi Jen.
goodtosave started a thread about it a while ago. Basically they are an ultra-safe short-term investment, and the interest rate is very good. Plus as I mentioned, the interest is exempt from state and local taxes, so if you live in a state like California, it can be a really good deal.
TreasuryDirect has a
FAQ and there is also a link to a Guided Tour to show you how it works. Investments are done electronically -- you can link your TreasuryDirect account to an account (usually savings), so when you buy T-Bills, your account gets debited, and then when the T-Bill matures, your account gets credited. For example, let's say you want to buy a $1,000 T-Bill. Your account may be debited, say, $970. And then 28 days later, your account will be credited $1,000. You've earned $30 in interest. You can only buy T-Bills on certain days, and the interest rate fluctuates (but has been very competitive with high-yield savings accounts).