When you create a slide presentation in Google Drive, you have a choice of themes and layouts provided by default.
Most monitors and displays are now in widescreen format, and presentations will by default be generated this way. You can, however, change the slide size by using the drop down menu in the theme chooser (or by using File > Page setup… from the top menu.
You can also edit any of these themes and layouts to suit your needs; add your corporate logo, change text layouts, add images and so on.
For each theme, editing the Master slide changes the defaults for all text styles used throughout your presentation. The background of the Master slide, for example, will be the default background for all slides, irrespective of their own layouts. If you want a corporate logo or background image to appear on every slide, you can apply it to the Master slide.
The remaining Layout templates control how your remaining slides will look. You can also add further layouts by clicking on the New Slide button (at the left of the toolbar), and rename or duplicate current ones by right-clicking on the layout.