Trade is happened only through a broker registered in stockexchange.
The buy or sell order is passed to exchange through a broker and the exchange finds counter party the the trade will be executed.
we can buy or sell any stocks in listed exchanges some charges will be applied.
Buying when the price is to raise is known as long and selling the same with profit is known as longunwinding.
Selling the price when the supply is more is called short and buting the same at low price is known as shortcovering.
Different types of trading are Intraday where the order may buy or sell must be executed or closed within a day.
Positional trading is meant for holding stocks for months or years and it comes under the category of investment.