Major

Since the Bretton Woods Agreement of 1944, the United States has been the global reserve currency, which makes the US dollar (USD) the currency of choice in international trade. It’s the most important of the majors.  

The euro (EUR) is the second most traded currency, followed by the Japanese yen (JPY), the pound sterling (GBP), the Australian dollar (AUD), the Canadian dollar (CAD), and the Swiss franc (CHF).

A major currency pair is the US dollar paired against any of the above currencies. 

Minor

The minor, or cross, currencies comprise all of the above major currencies when paired against each other without the US dollar.  

Exotic

The exotic currency pairs are those that feature the currency of an emerging market or less liquid currency in terms of global turnover on one side of the pair. 

Currency valuations are driven by many factors, chief among them probably being interest rate differences. A nation offering a higher interest rate than its trading partners will tend to see the value of its currency rising as investors sell low or non-yielding currencies for higher yielding ones.  

Then you have economic performance. A country exhibiting robust growth, with rising GDP, and a healthy labour market, will tend to have its currency outperform trading partners that may be experiencing economic contraction and unemployment.

A country that is comparatively more indebted, can also see its currency depreciate relative to a less indebted nation. These data are crucial to a certain contingent of FX traders who follow economic data releases closely and trade the volatility surrounding them. 

But currencies also exhibit other features and allow investors to gain exposure to the changing tides in other markets. For instance, currencies like the Australian Dollar (AUD), the Canadian dollar (CAD), and the New Zealand dollar are tightly correlated with certain commodities and so offer a means of exposing a portfolio to those assets without trading them directly. The Australian dollar is often traded as a proxy for gold, and the Canadian dollar for oil.

There are many benefits to trading forex as an asset class:

Unlike other markets, it trades around the clock throughout the working week.

Forex is also the most liquid market in the world, by far. Over $6.5 trillion is exchanged on the global currency markets every day. Forex is also the original decentralised market, long before crypto even existed.
 
It’s the lifeblood of the global economy and touches all other markets as every single transaction, whether it’s the procurement of commodities, the purchasing of bonds, or the trading of stocks, involves a foreign exchange component, especially if your national currency is not the US dollar. 
 
Finally, forex is a great training ground for traders. As well as touching all other markets, FX tends to be less volatile than other asset classes, so potential drawdowns tend to be smaller than in other asset classes.