In North America, a biscuit is a small, soft baked bread similar to a scone but not sweet. In some cases, it can be hard (see dog biscuit). In the United Kingdom, a biscuit is a small, crisp or firm, sweet baked good — the sort of thing which in North America is called a cookie. (Less frequently, British speakers refer to crackers as biscuits.) In North America, even small, layered baked sweets like Oreos are referred to as cookies, while in the UK, typically only those biscuits which have chocolate chips, nuts, fruit, or other things baked into them are also called cookies.
Both the US and the UK distinguish crackers, wafers and cookies/biscuits from cakes: the former are generally hard or crisp and become soft when stale, while the latter is generally soft or moist and becomes hard when stale.