What is msd

I've had a very slow computer tonight. I've not done much, some casual web searching, a google crossword puzzle, and then a game of downfall. There were a lot of programs running from earlier today including adobe reader and OOo, both of which seem to slow things down. I quit all and it got slower. I restarted and it was still slow. I looked at the activity monitor and something called mds was using 45-90% of the CPU. Its user was "root." My HD is 80G and is 77% full. While I was watching the monitor the mds stopped and now all is back to its usual speed.

What happened? Did I do anything to cause it? Is it a problem?

Thanks

Geoff