For actively traded stocks, the price could be set by the means of the supply and demand. If there are more customers than vendors for a specific stock, the price will rise. If there are more sellers than buyers, the price will fall. They have an inverse relationship. The economy, politics, or any catastrophic events could be the basis whether there will be more customers or vendors for a specific stock. If the events are momentous, it could have effect on or involve the entire market and move it up or down.