From BlenderWiki

[edit] Description

Blender has many options and features to make sure that you do not lose your work. First, it saves your actions in a list. At any time, you can tell Blender to back up in the list and undo most recent changes. Second, when you start Blender, one of the File options is to Recover Last Session. When you exit Blender, it saves the current file in a Quit.blend file; Recover Last Session merely loads that file back in. Third, you can tell Blender via User Preferences to automatically save versions "behind the scenes", and to keep old copies of your entire files every time you do manual saves.

[edit] Getting Started

By default, Undo is not turned off although it takes precious memory. To enable or disable undo, drag down your User Preferences window and click Edit Methods. In that panel, you may set:

• Steps: This numeric slider sets how many steps, or actions, to save. If you set this to 30, you will be able to undo the last 30 actions that you performed.
• Global Undo: This enables Blender to save actions outside of some mesh editing actions, for example, moving individual vertices while a mesh is in one editing session; each vertex move can be undone.

[edit] Undo

When you have done something terrible to your beautiful model, you have the following choices:

1. keep working forward and try to cover up or build on your accident,
2. Undo (via CtrlZ),
3. Revert to (open) a previously saved version in your working directory, or
4. Regress to the prior auto-saved version (if you have AutoSave turned on)

To regress to the last auto-save version, simply move your cursor toward the top of your screen until it hovers over the boundary of the User Preferences header and changes to an Up-Down arrow. Click and drag that header down to reveal the User Preferences window. Click the AutoSave tab, and click the "Open Recent" button. Immediately, the most recently saved work-in-progress version from the Temp directory will be loaded.

Warning Clicking the "Open Recent" button will immediatly load the most recent save, and you will lose any changes that you have made in the intervening minutes.

Upon loading the AutoSaved version, you may File/Save it over the current file in your work directory as a normal .blend file.

If you have versioning turned on (again specified in the User Preferences - AutoSave tab), each time you File->Save your .blend file, Blender pushes down the previous save to a .blend1 file, and the .blend1 to a .blend2 file, and so on up to the max number of versions that you have specified. To revert to one of those files, simply File->Open and select one of those .blendx files. Then File->Save it to bring it to the top of the stack.

[edit] Redo

Just as CtrlZ undoes an action, ⇧ ShiftCtrlZ re-does the last undone action(s).

[edit] History

AltU displays the Global History of what you have done as a list of actions named generally for what you did. Clicking on any action reverts you back to that state just before the next action was performed.