Collector's Items?

I've heard people talk about "collector's items" on these forums. I assume that these are items, but what makes an item a collector's item? Unless it's something you get from a collector , then I totally get it...

/Thanks in advance

You totally got it..

ROFL, thanks for telling me so I didn't look like a total idiot .
Shoulda guessed... oh, wait, I did .