Here is a brief explanation of the Time in Force option in the Advanced order form:
GTC: Good-till-cancelled - the order will remain active until it is filled or cancelled.
GTD: Good-till-date - the order will expire at the specified date & time if not filled or cancelled earlier.
IOC: Immediate-or-cancelled - requires all or part of the order to be executed immediately. Partial fills are supported, with any unfilled portion being cancelled.
FOK: Fill-or-kill - requires that all the orders be executed immediately. If this is not possible, the order is cancelled.