mysql rows

Hi, I feel a bit silly asking this. I don't think I quite understand how to apply functions such as mysql_num_rows() or MySQL commands such as COUNT().

Obviously I'm having trouble with some code, but I want more general answers before working on the code.

Can anyone explain how to count the number of rows from a table which have a particular value in a particular column? What if one or both of those are variables?

Thanks in advance. :)

Cross-posted to mysql.