a spider jockey is a kind of mob that appears as a skeleton riding a spider. It is probably the rarest common mob as you do not find them very often.the skeleton and the spider drop 5 exp points each. they usually appear in groups of 5 and are reasonably hard to kill.
a Ghast is a mob that lives in the nether but like every mob it can come through portals in to the normal world and destroy everything with fire balls.they drop ghast tears wich are used in potions. you can kill a ghast with a sword a bow and arrows or snowballs.