A transition is the effect that is used as a
slide comes onto the screen. To include transitions, first
create your slide show and then go to the Slide Sorter View.
(This can be found in the bottom left corner of the
PowerPoint window)
At the top of the window you will find a
transitions button on the slide sorter toolbar

Selecting the Transitions button will open
a Transitions pane on the right side of the window.

A preview of the transition which you
choose for a specific slide will take place on the slide
sorter view.
Always test your presentation before
showing it. What sounded like such a good choice while you
were writing, may not work out as well as you thought.