1) many posts say the requirement for CPA exam is 150 credit hrs. and i also read in many other places that the requirement for the exam is 25 units of accounting and 25 units of other related areas. what are the difference between them?
Significant majority of the states in US require 150 hour education requirement for certification. Out of this 150 credits (varies by state) specific number of credits must be in accounting or taxation from accredited programs. (that must be why you are reading 25 units of accounting etc)
2) as i was researching for macc programs, many require an accounting background. if the main purpose of macc program is to fulfill requirements for cpa exam, why would accounting undergraduates pursue this degree? does an undergraduate in accounting major not satisfy the req. for cpa exam?
Some require ACCOUNTING background while other don't. It depends on the program. Many great programs don't require accounting major as undergrad, but business major. Otherwise, there are additional courses you have to complete and if that is true, one might as just go for MBA in Accounting.
Undergrad accounting majors need 150 credits. 4 year undergraduate accounting program will probably have 124-130 credits. To get additional credits for 150hr requirement, many undergrad accounting students go onto MS program in Accounting or Taxation to meet this requirement. For audit, all Big 4 firms now pretty much won't hire anyone who can't sit for CPA exam by the time of employment.
3) if one obtains a macc degree, does he/she get one of the two years of experience required by cpa licence out of the way?
If one satisfies 150hr requirement through 4 yr college degree and MS degree, then in some states, your experience requirement is reduced to 1 year from 2 years. This depends on which state you are in.