This document provides an overview of brain-computer interfaces (BCI). It defines a BCI as a direct communication pathway between the brain and an external device. It discusses early BCI work with monkeys in the 1970s-1990s. It describes the basic mechanisms of how BCIs work, including different types (invasive, partially invasive, non-invasive) and applications. Examples discussed include controlling prosthetics and communication devices. Potential future applications are outlined, along with current limitations and ethical considerations of advancing BCI technology.