A bid is what people want to buy at and an ask is what people are willing to sell at. When a bid price equals a sell price, a trade is made.
You can sell to the highest bidder or, if you feel that his offer is too low, you can set your own price and your order will show up in the orderbook until someone buys at that price. The same thing for buying.
Check the
MtGox orderbook for example. They have much more volume than Intersango...