Accessing database statistics

Hi,
I need to write a query that returns the number of rows currently stored 
in a table. I presume that I issue a query against the system tables, 
but don't know which one(s) to do this against. So a couple of questions:

1) With specific reference to my problem, how do I do this?
2) More generally, is there any where that gives detailed descriptions 
of the system tables?

Many thanks,


Tony


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