If your order is "At Limit" they will only fill at the specified price you wish to purchase and they will buy you as many shares at that price as is on offer. So you may only get half or 3/4 of your order and the rest will wait for another seller at that price.
If your order is "At Market" you will be buying at whatever price the sellers are selling for and your whole order will likely be filled straight away (provided there are enough sellers) albeit at a price that the sellers decide. Needless to say this can be a risky way of buying stock.
