All slides are created using a design template (PowerPoint 2003 and earlier) or design theme (PowerPoint 2007) in your presentation. The default design template or theme, when you first start a PowerPoint presentation, is a plain, white slide. This plain, white slide is created from the slide master. All slides in a presentation are created using the fonts, colors and graphics in the slide master, with the exception of the title slide (which uses the title master). Each new slide that you create takes on these aspects.
Many colorful, preset design templates are included with PowerPoint to make your presentations more interesting. To make global changes to your slides, edit the slide master rather than each individual slide.
See also - About PowerPoint Slide Masters