What are stock markets and why is everyone talking about them?

After claiming that other countries are treating the USA unfairly when it comes to business, President Trump has brought in extra taxes for people bringing goods into America to sell.

They're called tariffs and President Trump says they'll help businesses in the US grow, but other people disagree.

His plans have led to big changes in the world's stock markets and to the value of shares in major companies.

But what is a stock market and how do shares work? De-Graft can explain.