A silly analogy might be helpful.
Imagine, for purposes of analogy, the CPU as a handheld calculator.
Early calculators were very similar because they just had basic functions, add, subtract, multiply, divide, maybe a square root function, etc.
But as calculators got more complicated, their keyboard layout changed -- and the way that the calculators expected YOU to enter complex equations became variegated as one calculator manufacturer or another thought he had a 'better way' to interact with the user.
In the analogy above, you, the user are like the OS. You know what you want to do -- but you need to be able to break that down into the simple -- and the not so simple,
complex instruction sets that the chip makers tend to build into their chips.
We've seen a move to change the way we approach those instruction sets, with newer generations
tending toward smaller instruction sets -- but with other interior architectural changes that balance this apparent reduction in complexity.
A site like
www.Anandtech.com will provide you more information on chip design and comparisons between the main families of chips from the major manufacturers than you'll probably ever want to know.