Copy slides between presentations

Open both presentations and switch to the Slide Sorter view.
Select the slide(s).
Choose Edit → Copy.
Switch to the destination presentation.
Click where you want the slide(s) to appear--before the first slide, after the last slide, or between two slides.
Choose Edit ? Paste.
The slide appears and adopts the design of the master in the current presentation. This is generally seen as a good default, since it allows you to insert slides from multiple presentations into a single presentation and yet to maintain the consistency of the single presentation's design.
If you want the slide to maintain the design from the original presentation, click the Paste Options button that appears and choose Keep Source Formatting.

If you choose to Keep Source Formatting, the slide's design (the slide-title master pair) from the source presentation is added to your presentation, so that the formatting of the slide can be maintained.