Keynote is an Apple's presentation software. Presentation software is primarily used for composing "slides" for presentations. Emphasis is on graphics and animation. Keynote files can be saved as PowerPoint.
PowerPoint is commercial presentation software that is part of the Microsoft Office suite. PowerPoint presentations consist of a number of individual pages or "slides". Slides may contain text, graphics, sound, animation, movies, and other objects. The presentation can be printed (with or without speaker notes), displayed or projected live by computer, or exported as an animated movie. Slides can also form the basis of webcasts.