What's going on with the Dow Jones right now?

Could someone simplify what happened today with Merrill Lynch and the Lehman Bros. etch? I'm a bit confused, always interested in economics, but making sense of it all is a bit difficult. I'm aware the housing market isn't helping the economy and the US govt. is now the number one mortgage holder after acquiring all the banks, but as you can tell my knowledge is minimal. What exactly happened? Good, bad? Thank you!