PowerPoint uses "masters" to store underlying design elements, such as placeholders, fonts, and background graphics. For example, if you want to put your logo on every slide, you'd put it in the slide master instead of inserting it on each slide individually. You also would make any global font changes in the master, rather than changing each slide. Intelligently using masters can save you a tremendous amount of time.
For years, PowerPoint users whined that you could not have more than one slide master in a presentation. However, in PowerPoint XP and later versions, now you can use multiple masters in one presentation. This enhancement dramatically increases your design flexibility. For example, now you could make different sections of your presentation have different background designs.
In any version of PowerPoint, you access the masters from the View menu. For example, to go to the slide master, you choose View|Master|Slide Master. But now in newer versions, when you are in Master view, you have an extra toolbar and panes. If you want the plain vanilla PowerPoint slide master with the default styles (like the "blank slide" template), click Insert New Slide Master on the Master toolbar. You can then customize this master to your heart's content.
If you want to use a master from a design template instead, again while you are in Slide Master view, click Design on the Formatting toolbar. A pane appears with thumbnails of the various design templates you have available. When you click a thumbnail, you see a small down arrow on the right-hand side. If you click the arrow, you see that one of the options is Add Design, so you can include the design in your presentation.
After you return to Normal view, your masters are available any time you need them. After you add a slide, you can choose to apply a different layout by choosing Format|Slide Design. Once again, the pane appears with your available designs. Click the down arrow and choose Apply to Selected Slide. If you click Apply to All Slides, the new slide master design replaces the old one on all your slides.