lock up 904 Question

I hate to divert from the interesting posts about the futue of Mopar under the facist boot of the Italians, but I gotta question.
My knowledge of anything modern Mopar (after 1972!) is virtually non existant.
So, my '87 /6 shortbed has a bad transmision. I'm going to take the time to make a swap up to a 318 as I take out the trans. (I have had the worse luck with /6's of anyone I know, and I've not been able to keep this one running well, ever. However, I do know it is not the same engine the truck came with- its and older model /6).
Anyway, my local Mopar used parts guy has a good small block 904 out of an '87 Diplomat or something, and it's a lockup.
I don't even know what the hell that means.
Can someone give me an education in simple terms?
What is good about it, bad about it, etc?
Author: storm king