Here are the differences:

Google has numerous top computer engineers working for them
in an ongoing basis. They are always tweaking, reprogramming, etc.
Search results are their bread and butter. Google must do this,
as webmasters all over the world are trying to conquer them.

Yahoo is still an old fashioned internet portal, with search on the
side. They are less militant. They are not overly concerned with
quality search results. They are more concerned about current
events, topics, news, etc. People on yahoo search for different
things than people on google.

Google and yahoo both sell the same main search items,
but in different times and different ways.

Target and Walmart sell the same items. But what items are
you going to just buy at Target? Walmart? Each store knows
its customers and knows which items most of their customers
come there for. I guarantee Target and Walmart will have the
same shampoos on the shelves, but their shelves will not put
the same items in the same places. Okay, not a great example,
just an idea.

Paul