Answer: Retrograde amnesia is a loss of memories that have been already formed, while anterograde amnesia is an inability to form new memories.
There are two defined forms of amnesia. One form of amnesia, called retrograde amnesia, is the loss of previously formed memories. For example, a person with retrograde amnesia may forget autobiographical facts about their childhood, such as family vacations or major life events such as moving or a death in the family. Retrograde amnesia may occur in Korsakoff syndrome, a type of amnesia resulting from thiamine deficiency, as can be seen in severe chronic alcoholics.
The other form of amnesia is anterograde amnesia, which is the inability to form new memories. Often times, these patients will not be able to recall events which happened minutes ago. They may still maintain their childhood memories, for example. They also still have their procedural or implicit memories intact - they can learn how to perform new motor tasks, for example. The most famous case study of anterograde amnesia is Patient HM, the man who had his bilateral temporal lobes removed to cure his intractable epilepsy. He was studied in depth by Dr. Brenda Milner. Another patient with anterograde amnesia is Clive Wearing, the British musician.
It is believed that older memories are stored in more places across the brain, and is thus more resilient to memory loss.