Here are some deferments and exemptions by classifications:
1-A Fully available and eligible for military service. All draftees are automatically placed in this category unless Selective Service (SS) determines otherwise.
1-A-O: Conscientious objector - One who is opposed to combat training and military service requiring the use of arms - serves in the military in a non-combat role (MASH unit medic, cook, supply sergeant, etc.).
1-O: Conscientious objector opposed to ALL types of military training and service - fulfills his service requirement via alternative civilian service jobs (Note: draftees must serve for a minimum of 2 years).
2-D: Ministerial students - deferral
3-A: Hardship deferments - one who if served in the military would provide EXTREME hardship upon his family AND the family would have no other means of support, not even relatives or friends.
4-B: Candidates of or elected government officials - deferral
4-C: Alien or duel national: Exempt depending on circumstances
4-D: Ministers of religions - Exempt
(can't remember classification): Surviving sons & sole surviving sons - this applies only to surviving sons who had fathers and grandfathers who died in the military from war-related injuries, not merely those who if they died couldn't carry on the family name.
4-F: Physical disqualification - can be a deferral or exemption depending on circumstances. Keep in mind that many men who could have been classified a 4-F were drafted illegally.
The penalty for not REGISTERING for the draft is a $250,000 fine and a 5 year prison sentence, plus ineligibility for any Federal benefits and jobs, including Financial aid. Offhand, I am not sure if that penalty or another one applies to one who REFUSES INDUCTION or ignores a draft call.
As far as Muhammed Ali's story goes, I'll leave that for another GCer to tackle.
A good site to check out for more info is:
Selective Service Website
Hope this helps