When doing a jab, you punch toward the front of whatever object you are punching and quickly retract your arm back into gaurd - as Kathy explained above.
*As a side note, with any punch you should always get your arms back into gaurd as quickly as possible to protect your face/head.*
With a hook, the focus of power would be to the side of someone's head. Bring your arm around rather than up (upward would be an upper-cut) and stop the punch when your hand has come halfway, or just past halfway around. Imagine jacking someone in the side of their head or face (for lack of a better example, I'm really not a violent person!) with a hook, whereas you would punch them in the jaw or nose with a jab... hopefully I'm making some sense. I'm no expert either but I did take a couple years of Tang Soo Do and we did learn hooks, uppercuts and jabs in addition to others.