Where did you learn to unit test? How did you learn?

[David Broadwell]

> I've noticed in my development that Python programmers (in general)
> write more unit tests than their java counterparts.  If I download a
> project off of jakarta.apache.org, it very rarely has unit tests.
> However, when I download a python module, it frequently has unit
> tests.

For the less educated here(spreading the bug early);  What exactly is a unit
test? How does the investment payoff? Where can the newer programmers find
good examples of unit tests?

( I personally know the answer to some of these, but answers should be good
for the group )