An AET file serves as a reusable After Effects template, intended for repeated use so you open it, save a new project, and customize that copy, while the template stores the construction plan of the animation including comps, timelines, layers, keyframes, effects, expressions, camera/light settings, render configurations, and all the folder/interpretation organization that holds the project together.

An AET normally doesn’t pack raw assets, instead storing references or paths to external audio, images, and video, which is why template sets are bundled as ZIPs with Footage/assets folders and why After Effects reports missing items if things were not packaged, and because AETs might rely on certain fonts or plugins, opening them elsewhere can trigger font swaps until everything is installed, and since file extensions aren’t exclusive, checking "Opens with" or the file’s source location is the best way to confirm the program behind it and what extra files should accompany it.

An AEP file is your ongoing After Effects project, while an AET is a reusable template, so in practice the difference lies in purpose: you open an AEP to continue that same project, but you open an AET to start a fresh file so the original stays clean.

That’s why AET files are regularly chosen for template-based motion graphics (intros, lower-thirds, slideshows): the master AET stays unchanged while each new project starts by opening it and doing a Save As to create your AEP, where you modify text, colors, logos, and media, and although both formats include the same elements—comps, layers, effects, keyframes, expressions, cameras/lights, and settings—and both rely on external footage paths, the AET is meant for safe templating and repeatable output, while the AEP is the editable project you keep refining.

An AET file mostly contains the framework and behavior of a motion-graphics project rather than the footage itself, offering compositions with resolution, frame rate, duration, and nesting, and keeping the entire timeline of text, shape, solid, adjustment, and precomp layers, with properties such as position, scale, rotation, opacity, masks, mattes, blending modes, parenting, and all animation elements including keyframes, easing curves, markers, and optional expressions.

Beyond that, the template includes your effects and their parameters, from color correction and blurs to glows, distortions, and transitions, as well as any 3D configuration with cameras, lighting, and 3D layer options plus render/preview settings, and it also preserves project organization like folders, label colors, and interpretation settings, though it usually doesn’t pack raw media, audio, fonts, or plugins—only file paths—so opening it elsewhere may cause missing-footage or missing-plugin alerts until dependencies are restored.