What's the difference between unconscious and subconscious in freudian psychology?

Buried vs. Cache. The unconscious in freudian psychology refers to repressed mental information that is not easily brought to the conscious mind, if at all. It is stored away far below the surface. The unconscious protects us from memories that are too overwhelming to us, whether this is reasonable or not. The subconscious is like cache in a computer--it's readily accessible if we want to get to it.
Freudian Psych. The unconscious holds repressed memories and experiences, as well as unpleasant feelings. These things are out of our awareness unless brought up via psychotherapy and/or daily experience. The subconscious is sort of a mid-level, in which some of these "less charged" things are available through cues or clues. An example would be the 'tip of the tongue' phenomenon.
Layers . Subconscious and unconscious refer to layers of awareness. When something is in your subconscious you may be aware of something going on, shall we say, but you're not able to put words to it. It's there, but it's vague, and undefined. When something is in your unconscious, there is no awareness of that thing whatsoever.