I have a table that looks like this:
FOO BAR BAZ
----+----+----
foo1 bar1 baz1
foo2 bar3 baz2
foo3 bar1 baz3
foo4 bar1 baz4
foo5 bar3 baz5
foo6 bar1 baz6
f
SELECT foo, count(bar) FROM mytable GROUP BY bar ORDER BY count(bar) DESC;
The group by statement tells aggregate functions to group the result set by a column. In this case "group by bar" says count the number of fields in the bar column, grouped by the different "types" of bar.
A better explanation is here: http://www.w3schools.com/sql/sql_groupby.asp