You can find the basic difference in google, however for the latter question about functionality in tableau 10 please refer below.
In previous Tableau versions, you needed the Data-Blending solution to join data from different databases. This feature works well enough in one-to-one relationships, but unwanted asterisks pop up when we want to perform a join in one-to-many relationships.
The functionality implemented in tableau 10 was 'cross database join' which allows us to cross data between different sources easily which is similar to blending.
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I had the very same questions in my mind initially.. I found this explanation at a very basic level comparing to Excel very convincing...
Another couple of very concise but clear explanation on Joins Vs Blends...
This explanation on Tableau site is also worth throwing a glance
As Kishore mentioned there are a many more informative sites that will come up on googling. But these two I had found very useful to clear my foggy mind on when to use what..
Hope it helps...
Thanks a lot. Its very helpful.