'Slide Masters' are the templates or layouts, which are shown in in your 'new slide' button:
These can be modified, duplicated or created from new, so that you can include your own template slides to work from. Slide Master can also contain logos or images, as well as pre-defined Text which remains un-editable when viewing the slide in regular mode (not editing in slide master).
It is also possible to add content placeholders for text, images, SmartArt and any other type of content within PowerPoint and these can be added to new Slide Masters or existing ones. These placeholders ensure you are using the same formatting and layout and spacing between slides with minimal effort.
Accessing Slide Master
In the view Tab of the menu ribbon you will find a 'Slide Master' Button.Clicking this button will begin editing the template slides (Slide Masters) within that PowerPoint file:
Once in this mode, you will notice an extra 'Slide master' tab in the menu ribbon. In this tab you can create, rename or change the theme of any of the current Slide Masters. You can also add Placeholders for all types of content. It also contains the 'Close Master view' button to return to regular editing within PowerPoint (your Original presentation):
Choosing which slide master you want to edit
Once in Slide Master view, before adding any content or placeholders first decide if you want to apply this to all slides or just one type of layout:
Master (All slides) - The master slide at the top will represent the style of all the layouts nested below it, changes made to the 'Master Slide' are shown on ALL layout sides below.
Layout slides (Individual)- All slides below the master slides are layout slides. These layout slides can be created from new or modified individually for each slide you see in the "new slide button" to begin with. Any changes made to these slides will only be represented on that individual layout.
To add content that will feature on a slide layout frequently e.g. a name, title, logo or something you expect to use often without editing.
Select the relevant layout/slide you with to add the content to,
In the Insert tab of the menu ribbon, select the type of content you wish to feature in your layout:
In the 'slider master' tab of the menu ribbon you will find a 'Insert placeholder' button when a layout slide is selected (greyed out when the Master or parent slide is selected).
Use this button to place a content placeholder on your layout slide. When you create a new slide with that layout/template, the content placeholder you created will be in the same location every time and won't be filled with content, unlike when duplicating a slide.