Keynote is the presentation software part of the Apple iWork suite. It allows you to create slides and combine these into a presentation. You can include different kinds of material: text, images, diagrams, sound, video. Keynote will allow you to control the design of the slides and the transition between them in many different ways.
Learning to use Keynote is not only about creating slides and making the slide show run. You should also think about what you put on the slide and how you present what you want to say in a professional and effective way. The IT Learning Programme runs a series of courses on presentation software and related issues - such as how to use it to successfully get your message across. See the ITLC course catalogue for details.
Similar tools are the free Open Office Impress tool, see www.openoffice.org/product/impress.html and Microsoft PowerPoint. Keynote can be used to open and save PowerPoint files, and you can also save your Keynote files in PowerPoint format.