Stocks that trade via OTC are typically smaller companies that...

Stocks that trade via OTC are typically smaller companies that cannot meet exchangelisting requirementsof formal exchanges. However, many other types of securities also trade here. Stocks thattrade on exchangesare called listed stocks whereas stocks that trade via OTC are called unlisted stocks.

Can you give a simple explanation about what is OTC?
I've looked through internet, but I could not have a good understanding.
How can someone buy a stock from companies that are in this over-the-counter listing?
It says that it is for companies that can not meet exchange listing requirements to trade on ASX, isn't Aziana listed on ASX?

Thank for helping out