Exchanges and stocks - amateur question

Hi. Amateur question here.

I've been 'messing around' with stocks abit, using TradeKing.

Recently I had the chance to see Interactive Brokers in action and the question is this:

In TK, when you buy a stock, say GOOG, you simply... click buy.

In IB, you can choose the exchange.
Example:
GOOG Stock SMART USD NMS
GOOG Stock ARCA USD NMS

and so on.

What is the difference? What is going on that I cannot see in TK?

Pardon the ignorance,

Dane
 
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