Bid price is the point at which a buyer is ready to buy. Ask price is the value point at which the seller is ready to sell. When the two value points match in a marketplace, such as when a buyer and a seller agree to the prices being offered by each other, a trade takes place.
These prices are determined by two market forces -- supply and demand, and the gap between these two forces defines the spread between buy-sell prices, this is called bid-ask spread. The larger the gap, the greater the spread.