A cookie (or biscuit in the UK) is a small flat or slightly raised cake. In the USA, cookies can be large and chewy in texture, or crispy. The name 'cookie' derives from the Dutch koekje, which is a diminutive of koek, cake. Popular types of cookies in the US include Oreos and chocolate chip cookies.
In the UK, a cookie is not usually called a cookie. It is called a biscuit, from a French word for twice cooked bread. In the UK biscuits are generally small and crispy in texture. The UK term for a US-style biscuit is a scone.