When memories are encoded in the brain, they are stored as connections between neurons known as neural networks. The more information is rehearsed, the more this neural network is activated, and the stronger and deeper the memory becomes. Without use, the neural connections weaken over time. This process, known as decay, explains why we forget information. The longer the neural network goes without activation, the more the connections between neurons decay, and the more difficult it becomes to access the information.
Forgetting also occurs due to interference. When memory is written into a neural circuit, the neurons do not suddenly deactivate and devote themselves solely to storing that information. When new information is brought in, it can encode over the old memory's neural circuitry, leading to forgetting of old information due to interference from new encoding.
Proactive interference is when old memories inhibit the encoding of old memories, such as forgetting your new address because you keep remembering the old one. Retroactive interference is when new memories override the old circuits, such as forgetting how to play an old song on an instrument after being taught a new one.
Amnesia is memory loss where one cannot recall or encode information. In retrograde amnesia, past memories that were previously encoded cannot be retrieved. This is generally caused by diseases such as dementia. Anterograde amnesia is when one cannot form new memories and is unable to encode new information. This is generally caused by brain damage to the hippocampus region, although it can also happen temporarily due to blackout from alcohol over-consumption.
Diseases such as Alzheimer's lead to memory loss because of its effects on the brain. With Alzheimer's disease, neuronal connections degenerate, especially in the hippocampus area of the brain. Without these connections, memories cannot be retrieved because the neural circuitry they are encoded in is no longer functioning.