ERASE: This command removes one or more entities (objects) from drawing. Command: ERASE (Return) Select Objects: (Apply any of the selection sets)
OOPS: Oops command allows you to restore the objects at their original location. Command: OOPS (return)
Delete: After selecting unwanted entities from the drawing you can directly erase them using the delete key on your keyboard.
UNDO: The Undo command reverses the effect of many AutoCAD command previously used. Command: UNDO or U (Return)
Note: UNDO has no effect on some commands and system variables, including those that open, close, or save a drawing, display information, change the graphics display, regenerate the drawing, or export the drawing in a different format.
REDO: REDO reverses last UNDO command. UNDO can be used any number of times till beginning of the drawing, but REDO can be applied only for the last UNDO.