What is the difference between the theme Slide Master and a layout master?
For every layout in presentation, the layout master is associated with the theme Slide Master.
Theme Slide Master, which is the primary slide master, contains text, graphics, and formatting on all slides of the presentation. And if the formatting for a single slide is changed, the formatting for all the slides will be changed.
Whereas in the layout master, if the title and content option are applied, the layout is changed for that slide only, and it will not affect the other slides.
The changes made in the theme Slide Master will change all the slides in the presentation. Whereas in the layout master, the changes done will affect the slides only in which the same layout is applied.