cassette question

Hi ladies,
okay, here is a somewhat random question. what is the difference between a cassette and a freewheel?

i've heard them used interchangeably, but i've also heard people mention how they had to get a new hub because before they had a freewheel and now they have a cassette, blah blah blah.

is there a difference?

thanks! :)

Here's a link to some nice info. I have a road bike that still uses a freewheel. The freewheel is heavy but it is great in that it allows me to play with the size of the cogs. For instance, one year when my knees were really bothering me, I put a huge cog on the inside that allowed me to climb cliffs. It looked odd and you had to be careful shifting, but it worked. Later I switched a few more of the cogs so I could keep this huge cog but smoothly ramp down to the normal 11 or 12 on the outer.

http://www.sheldonbrown.com/free-k7.html

thanks! that's really interesting.... now i want to go take a look at all the old bikes in my parents' garage and see how they're made! ;)