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Previewing animation
After Effects provides five options for previewing your animations: OpenGL interactive preview, RAM Preview, standard preview, manual preview, and wireframe preview. OpenGL preview is the default preview option if you have the necessary OpenGL hardware installed. You can enable and disable it as necessary by using the Dynamic Preview button 
on the Composition window or by changing the OpenGL preference. You can access RAM, standard, and manual previews using the Time Controls palette. Access wireframe preview from the Composition > Preview menu.
After Effects also uses caching and dynamic resolution to facilitate quicker previews and Composition window updates whether you're using RAM Preview, using OpenGL interactive preview, advancing sequentially or nonsequentially through the frames, or dragging layers in the Composition window. Wireframe mode is also available to speed up Composition window updates. (See Previewing 3D for information on using Wireframe mode.)
The preview options represent different balances between preview quality and speed:
OpenGL interactive preview
This option plays a preview of the frames by using installed OpenGL hardware. During OpenGL preview, memory is supplied by the OpenGL hardware and frames are only displayed onscreen rather than being cached, so the quality is high and the speed is exceptionally fast. Use OpenGL preview to interactively preview footage in the Timeline window by dragging underlined values, scrubbing the timeline, or dragging footage in the Composition window. This option is invaluable for client demonstrations when you want to show your work in a fluid manner without waiting for After Effects to render each change. (See Using OpenGL interactive previewing.)
RAM Preview
This option plays a preview of the frames (including audio) at the frame rate of your composition or as fast as your system allows. Use RAM Preview to preview the footage in the Timeline, Layer, or Footage windows. The number of frames previewed depends on the amount of contiguous RAM available to the application. After Effects displays the total number of frames requested for the preview and the number it can render. (See Using RAM Preview.)
You have two choices of RAM Preview settings: RAM Preview and Shift+RAM Preview. Each provides different options. In the Timeline window, RAM Preview previews either the span of time you specify as the work area or from the current time indicator forward. In the Layer and Footage windows, RAM Preview previews only untrimmed footage. Before you preview, check which frames are designated as the work area. (See Setting up a work area.) Before you preview the Layer or Footage windows, check to see that footage you may want to preview has not been trimmed.
Audio plays only for layers in which the Audio switch is on. (See Previewing audio.)
Standard preview
This option provides a preview of all frames in your composition. When you use this option, After Effects displays every frame as quickly as it can using the current settings of the layer switches, composition switches, and composition resolution. This preview generally plays slower than real time. Standard preview is useful when your composition is simple or in its early stages and doesn't require additional memory for displaying complex animations, effects, 3D layers, cameras, and lights. (See Using standard preview.)
Manual preview
Using the shuttle control, the jog control, and the current time indicator, you can manually navigate through a composition, layer, or footage file. The current time indicator shows the position of a frame relative to the beginning and end of a composition, layer, or footage file. Because the current time indicator travels only a fixed distance in the Time Controls palette, it is less precise with long compositions. (See Using standard preview.)
Wireframe preview
This option displays a preview of the frames for all layers in your active workspace. Each layer is represented by a rectangle or, if a still-image layer has a mask or an alpha channel created in another program, by the outline of the mask or alpha channel. (See Using standard preview.)
Adaptive Resolution preview
This option degrades the resolution of the layers by an amount you choose, so that they use less memory to display. Use this option when you want to preview complex or animated effects or when you want to preview movies and layers that require extensive memory. You can toggle this option on and off by using the Dynamic Preview button in the Composition window. (See Using Adaptive Resolution previews.)
You can also use the Motion with Trails preview, which preserves the outline of each frame of each selected layer so that you can see an outline trail for every frame. (See Using standard preview.)
Note: Previews use the frontmost composition view for 2D compositions and the Active Camera view for 3D compositions when multiple views are opened. To turn off the Active Camera, deselect Previews Favor Active Camera in the Time Palette options menu.
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