The default design template when you first start a PowerPoint presentation, is a plain, white slide. This plain, white slide and the font choices used on it were created in 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 master slide rather than each individual slide.