A slide transition is the process of changing from one slide to
another in a presentation. At its simplest, one slide can disappear
and the next one immediately appear. You can do many kinds of
transitions, like a slide moving in from one side and pushing the
other slide out. You can also set the amount of time a transition
takes.
A custom show is a slide show that you can make from your full
set of slides by picking some and displaying them. They can even be
picked in a different order. Sometimes you might have variations of
a slide show for different audiences. There might be some slides
that you show to some that you would not show to others. Instead of
creating several individual slide shows as separate files, you can
create one slide show and create a few custom shows from the set of
slides you have, without creating a new file.