How does this query work.....?

Hi,

I am using the following query to find the attributes of a given table, and
their datatypes:

select typname,attname
from pg_class c, pg_attribute a,pg_type t
where relname = 'table_name' and
attrelid = c.oid and
atttypid = t.oid and
attnum > 0;

Can anybody explain how this query actually works - I cannot figure it.
Thanks in advance.

Rob Burne.