noobie join question

Hi List,
I am having trouble trying to figure out
how to get the result listed at the bottom.

I have 3 tables units, types of units which has a description of the units,
and a table that list associations of the units. I can't figure out
how to do the proper joins. Any pointers would be appreciated.

create table types (
id integer,
descr varchar(30)
);

COPY types (id, descr) FROM stdin;
1 descr 1
2 descr 2
3 descr 3
4 descr 4
\.

create table units (
uid integer,
udevice varchar(30),
utype integer
);

COPY units (uid, udevice, utype) FROM stdin;
1 aaaaa 1
2 bbbbb 1
3 ccccc 4
4 ddddd 3
\.

create table assoc (
aid integer,
src_id integer,
dest_id integer
);

COPY assoc (aid, src_id, dest_id) FROM stdin;
1 1 2
2 1 3
3 3 4
4 4 2
\.

desired result
aaaaa | descr 1 | bbbbb | descr 1
aaaaa | descr 1 | ccccc | descr 4
ccccc | descr 4 | ddddd | descr 3
ddddd | descr 3 | bbbbb | descr 1

Thanks,
Steve