Here's what made outer joins click for me:

You have two tables:

Students -- studentid, lastname, firstname
FavColor -- studentid, colorname

You want to generate a list of students, and you also want their favorite color. Some of the students told us their favorite color, but some didn't respond. So only some students are in the FavColor table. If they aren't in there, we still want the student to be in the list, but just display a NULL for their favorite color.

select s.studentid, s.lastname, s.firstname, fc.colorname
from Students s
left outer join FavColor fc on fc.studentid = s.studentid

This pulls everyone from the Students table, and if we have a FavColor saved for them, it also shows that, otherwise it returns a NULL for that student.