Not a stupid question at all. With most exchanges going to electronic order execution, AMEX has moved to some kind of hybrid system with both electronic and live auction. NASD and NASDAQ are both electronic orders by market participants but I am not sure if one is specific to a particular type of participant. I am guessing one is just a symbol that represents all the ECN's best bid/best offer, I think NASDAQ might be that. If you choose to route your order through a specific ECN, lets say ARCA, it might show up as NASD.
Most platforms do this automatically to get you the best fill, but with many you can choose the routing you want. If you have AMERITRADE, I know you can do this, just click on the routing drop down menu and you will see several choices.
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