Difference between dpkg and other installs

[Steve Doerr <sdoerr@qni.com>]

Could anyone help me with the difference between dpkg and other
installs.  I have installed netscape and wp8 w/o using dselect.
Should everything be installed through dpkg to get the cleanest
Debian system or does it matter?  I also need JDK 1.1 and
PostGresql 6.5 or greater and I don't want to create conflicts
with libraries (i.e. libpgsql) that won't be identified outside
of dselect.  I don't think either of the above has the package
info needed to APT them, so they'll have to go on manually
(unless somebody can point me to some documentation on how to
run anything through dselect).  Any insight would be greatly
appreciated.
Thank you