A grease spell covers a solid surface with a layer of slippery grease. Any creature in the area when the spell is cast must make a successful Reflex save or fall. A creature can walk within or through the area of grease at half normal speed with a DC 10 Acrobatics check. Failure means it can't move that round (and must then make a Reflex save or fall), while failure by 5 or more means it falls (see the Acrobatics skill for details). Creatures that do not move on their turn do not need to make this check and are not considered flat-footed.
The spell can also be used to create a greasy coating on an item. Material objects not in use are always affected by this spell, while an object wielded or employed by a creature requires its bearer to make a Reflex saving throw to avoid the effect. If the initial saving throw fails, the creature immediately drops the item. A saving throw must be made in each round that the creature attempts to pick up or use the greased item. A creature wearing greased armor or clothing gains a +10 circumstance bonus on Escape Artist checks and combat maneuver checks made to escape a grapple, and to their CMD to avoid being grappled.
Can a creature take a 5-foot step within the area of a grease spell? I believe it can and heres why I think so.
Very often I've heard that because of the sentence "A creature can walk within or through the area of grease at half normal speed with a DC 10 Acrobatics check" That means that the Grease is difficult terrain and therefore you can't 5-ft step. It doesn't say that grease is difficult terrain, as it does in Web, Shifting Sand and Ice Storm. Furthermore walk speed has absolutely nothing to do with your 5-ft step, creatures with movement speeds of both 100 and 10 have the same 5-ft step distance.
Even if you can five-foot step (I'll let others argue that), you'd still have to make that DC10 check, unless you're making that step with your fly speed.