Trading is simply buying and selling of assets that could comprise goods and services where the seller is compensated by the buyer. In the financial world, these assets are called financial instruments and could be futures, stocks, margin products, derivatives, currency pairs, options, cryptocurrency, etc. 

Usually, the term trading is referred to as short-term trading where buying and selling occurs over a shorter span of time. There are many different categories in trading such as wing trading, day trading, trend trading, etc.