show provides the package details for the package, so name, version, description and various technical information.
showpkg provides information regarding
dependencies (ie what this package needs to already be installed to function)
reverse dependencies (what other packages rely on this one) and
provides which says what function the package provides. An example of this would be providing
Lets say you have a package
foobar that needs syslog running in order to run. You could install
syslog-flibble. If each of these 'provide' syslog, then
foobar can run regardless of exactly which one is installed.
Unless you are troubleshooting a dependency issue, then
show usually provides the most useful information.