[dev] id and UID

[Luis Felipe Marzagao/Andamentos]

Hello:

I've noticed many horde apps (nag, mnemo, turba etc.) have id and uid 
fileds in their database schema.

I'm trying to develop an in-house module and I'm trying to follow horde 
apps style and good practices so maybe my app could be added to horde in 
the future.

I'd like to understand why there's a need for both id and uid fields, 
since at first glance (at least for a beginner) the id field already 
seems pretty unique. Any pointers would help.

Thanks a lot.
Luis Felipe