Note - Click this link for tutorial on Slide Transitions in PowerPoint 2003 and earlier versions
Choose a Slide Transition
Click on the Animations tab of the ribbon to access the slide transition options.
The most common slide transitions are shown as icons on the ribbon. Click the drop down arrow to see the complete list of transitions available, or scroll through the list, one row at a time.
What's new in PowerPoint 2007 with regard to slide transitions, is that as you hover your mouse over a transition icon, you will see the effect shown on the slide. You no longer need to actually apply the slide transition to see the effect, until you have made your choice.