The PPTA file is a container created by Presenter '13 to store resources for your presentation, such as audio clips, videos, web objects, quizzes, interactions, and more. It has the same name as your PowerPoint file (with a file extension of *.ppta) and is located in the same folder. And it must be kept with your PowerPoint file at all times.
To backup your project, move it to another location, or share it with another developer, create an Articulate Package. Then move the resulting zip file to another location, or send it to another developer. For details, see How to Create an Articulate Package.
Here's a brief video screencast explaining the PPTA file and demonstrating how to create an Articulate Package: