[BioC] basic question regarding biomaRt

[Elizabeth Purdom]

Hello,
This is an extremely basic question regarding biomaRt. Where can I find 
careful descriptions of what the attributes mean? I've tried all kinds 
of searches, but I don't think I'm hitting on the right term. For 
example, I'm accessing ensembl and I've never understood the difference 
between some of the attributes I frequently.
Thanks,
Elizabeth