When you want all your slides to contain the same fonts and images (such as logos), you can make those changes in one place—the Slide Master, and they'll be applied to all your slides. To open Slide Master view, on the View tab, select Slide Master:
When you edit the slide master, all slides that are based on that master will contain those changes. However, the majority of changes that you make will most likely be to the layout masters related to the master.
When you make changes to layout masters and the slide master in Slide Master view, other people working in your presentation (in Normal view) can’t accidentally delete or edit what you’ve done. Conversely, if you're working in Normal view and find that you're unable to edit an element on a slide (such as, "why can't I remove this picture?") it may be because the thing you're trying to change is defined on the slide master or a layout master. To edit that thing, you must switch to Slide Master view.