An AET file is best known as a template for Adobe After Effects, designed so you can open it repeatedly and save new versions rather than overwrite the source, with the file storing everything that defines the motion graphic: comps, timeline structure, layer stacks, animation keyframes, effect setups, expressions, cameras/lights, render settings, plus organizational items like folders and interpretation settings.

An AET normally doesn’t store raw media; instead it holds references to external video, audio, and images, which is why template packs often come zipped with an assets/Footage folder and why missing-file dialogs appear if media gets excluded, and since AETs may require certain fonts or plugins, opening them on another system can trigger missing-effect errors until you install or relink what’s needed, with the added note that file extensions can overlap, so confirming the true source via "Opens with" or the file’s origin folder is the best way to know what program created it.

Here's more regarding best AET file viewer have a look at the webpage. An AEP file is the standard AE project where you build and edit animations, updated as you import footage, adjust comps, and refine effects, while an AET is a template meant as a reusable starting point, so the practical difference is workflow: you reopen an AEP to keep editing the same project, but you open an AET to create a new project so the template stays untouched.

That’s why AET formats are widely shared in motion-graphics template sets like intros, lower-thirds, and slideshows: the AET remains the creator’s master, and for each new video you open it, Save As a new AEP, then swap in your own text, media, logos, and colors, and even though both formats store the same project components—comps, layers, keyframes, effects, expressions, cameras/lights, and settings—and both usually reference external files, the AET safeguards the layout while the AEP becomes the editable end-user project.

An AET file retains the structure and animation logic of an After Effects project but not always the media assets, containing compositions with defined resolution, FPS, duration, and nesting, plus the complete layer arrangement—text, shapes, solids, adjustments, precomps, and placeholders—with layer properties like position, scale, rotation, opacity, masks, mattes, blending modes, parenting, and the project’s animation data including keyframes, easing, markers, and any expressions used to automate motion.

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.