What is the difference between "buy" and "purchase"?

Is there any difference between the word "buy" and "purchase"?

Well, purchase generally refers to something that's contractually bound, such as a land agreement, but purchase can be used to refer to buying something too, but you wouldn't "buy" land.

So basically, you can purchase anything, but you can't buy anything that's contractually bound.

I think, purchasing immovable things like land, building, etc is buying;
and purchasing small and movable things like pencil, pen, etc is purchasing.

If there is any difference at all it's that purchase reads a bit more formal. In current U.S. usage they mean the same thing. There are times where one just reads better than the other. How would you substitute buy for purchase in this sentence?

"The government announced the purchase of a new computer system."

I agree with Rob. I think that the only difference between the two is that purchase sounds more formal in certain sentences. Either way you look at it, you are walking away with an item that you've spent money on. Weather that money is paid now or later it doesn't matter. You are still paying for it one way or another.

I think we use 'purchase' for the meaning of making available for uses...... And buy is used to the things that are being atteched strictly with any financial concept.. About Purchasing we are not sure that this is against money.... If we use buy this is sure that the thing was purchased against some money.. So the purchasing reffers to make available however....

As verbs, they mean the same. Like Rob said, purchase is more formal. However for nouns, you can only use purchase. "The buy" is not incorrect but "the purchase" is fine.