select count(*) from <tablename> returns the number of rows 
in the table.

select count(col_name) from <tablename> return number 
values in the column (Ignores null value in the colum). For 
example if we execute the following 2 queries then

select count(*) form emp
select count(comm) form emp 

The result will be
14
4

for the First Qry,result is "No.of records"
for the second Qry,result is "No.of records for particular 
columnname"