In plpgsql: difference between := and =

I know I've seen this somewhere, but I've searched the docs and can't
seem to find it.

I know that := is assignment, and, from the docs, (and experience),
know that it does a "SELECT" to return the assigned value.

I was experimenting with triggers, in the body, I was trying something
like:

BEGIN
   NEW.name := get_who(NEW.name);
   RETURN NEW;
END

where get_who() is a stored procedure.

After going through some of my pgsql- mail-archives, I saw a similar
example where they used simply "=".

After I did this, it worked.

Could someone explain the why's and the difference?  I know I've seen
this explained before, but where, I don't know...