Most CAD users are familiar with how the Freeze/Thaw and On/Off commands work, but many probably don’t understand how they differ. Turning a layer off does indeed make it so that every object on that selected layer is not visible to the user. However, that layer can still be erased or moved; for example, if the user were to perform an Erase All or Move All operation. Turning a layer off does not protect those objects from alteration; it simply allows a user to visibly remove that particular layer from the user’s current view.
Freezing a layer also makes all objects on that layer invisible to the user, but it has a few other important benefits. If an Erase All or other similar operation is completed, it will not erase layers that are frozen. Another benefit of a frozen layer is that objects on that layer are no longer using system memory for operations such as regen, redraw, zoom, and pan, which can be a real benefit for drawings with very large objects or data sets.
To summarize, turning a layer off should be thought of as more of a temporary state in order to increase screen visibility for a particular moment. Freezing a layer should be thought of as more of a permanent operation for layers that a user may have no use for in the drawing other than design backup data, or that may inadvertently tag along within blocks or xrefs brought into the base drawing.
Notes from Cadalyst tip reviewer R.K. McSwain: This is a good review from Scott on the differences between Freeze and Off and what effects those differences may have on your drawings.