Repression (repressed memory)
When explaining repression, Freud compared the process to "condemnation" and stated the following: "Let us take a model, an impulse, a mental process seeking to convert itself into action: we know that it can suffer rejection, by virtue of what we call 'repudiation' or 'condemnation'; whereupon the energy at its disposal is withdrawn, it becomes powerless, but it can continue to exist as a memory. The whole process of decision on the point takes place with the full cognizance of the ego. It is very different when we imagine the same impulse subject to repression: it would then retain its energy and no memory of it would be left behind; the process of repression, too, would be accomplished without the cognizance of the ego."
Wow - what does all that mean? In a nutshell, Freud was saying that when we have memories, impulses, desires, and thoughts that are too difficult or unacceptable to deal with, we unconsciously exclude them from our consciousness (some people like to say we "push" them down from our consciousness to our uncosciousness). This is similar to suppression with one key difference - suppression is a conscious exclusion (or "pushing" down) of these painful memories, thoughts, etc., and is more similar to Freud's explaination of condemnation.