What is In Angle's classification for malocclusion, key tooth is: and explain in detail?
Angle’s postulate was that the upper first molars  were the key to occlusion and that the upper and lower molars  should  be  related  so  that  the  mesiobuccal  cusp  of  the  upper molar occludes in the buccal groove of the lower molar. If the teeth were arranged on a smoothly curving line of occlusion and this molar relationship existed , then normal occlusion  would result. 
 
Angle then described three classes of malocclusion, based on the occlusal relationships of the first molars:

Class I: Normal relationship of the molars, but line of occlusion incorrect because of malposed teeth, rotations, or other causes.
Class II: Lower molar distally  positioned relative to upper molar, line of occlusion not specified.
Class III: Lower molar mesially positioned relative to upper molar, line of occlusion not specified.

Reference: Contemporary orthodontics, Proffit 6th ed page no 2