Retrograde amnesia refers to the type of amnesia that occurs when you can't recall anything before the onset of your amnesia. For example, one day you wake up after suffering some sort of trauma and you don't know who you are are where you came from. From that day on you continue to make new memories, and can even relearn who you were before but you won't actually remember it. Anterograde amnesia is when you can't remember anything after the onset of your amnesia. A very popular example of this is the movie 50 First Dates, in which Drew Barrymore's character reverts back to the day that she suffered her trauma every 24 hours. The amount of time that it takes for the memory to reset is different with any case, but scientists postulate that whatever trauma caused the amnesia somehow disrupted the memory storage processes so memories can't be transferred from short term memory to long term. The memory consolidation theory basically states that memories become more stable over time because they go through several biological processes including glutamate release and protein synthesis. When memories go through this process you are less likely to lose the ability to recall them.