The hippocampus is a part of the forebrain, located in the medial temporal lobe. It belongs to the limbic system and plays major roles in short term memory and spatial navigation. Humans and other mammals have two hippocampi, one in each side of the brain.
Brain function depends on inhibitory cells that balance or 'brake' excitation. These neurons allow the brain to process information and also prevent runaway seizures.
Degenerative diseases such as Alzheimer's often involve complex interactions between multiple proteins and other biomolecules.
Imagine you are meeting a friend for dinner at a new restaurant. You may try dishes you haven't had before, and your surroundings will be completely new to you. However, your brain knows that you have had similar experiences -- perusing a menu, ordering appetizers, and splurging on dessert are all things that you have probably done when dining out.