Ultra ATA/100 / Ultra ATA/133??

[oso_fast]

whats the main difference between Ultra ATA/100 and Ultra ATA/133..im asking because someguy is trying to sell me a Ultra ATA/100...and i was wondering if it makes a difference if its Ultra ATA/100 or Ultra ATA/133...

also if i have a Ultra ATA/133 would i be able to connect a Ultra ATA/100 to it?

thanks in advance