This is what the Aseprite workspace looks like in the Default layout:
The most important elements are the sprite editor, the timeline, the color bar and color pickers, and the preview window. Other elements are the menu bar and the tool bar. The status bar also gives you valuable information about the current sprite.
The timeline is not visible by default; it can be made visible with View > Timeline or the Tab key (It also pops up automatically when you add a new frame or a new layer).
See the Workspace Layout section.
You can drag-and-drop tabs to any side of the sprite editor window or duplicate them using the Ctrl or Alt keys along with the Left Mouse Button:
SEE ALSO
Color Bar | Context Bar | Sprite Editor | Status Bar | Tabs | Timeline | Tool Bar | Menu Bar | Workspace Layout