Let's start with the basics...
Buying and selling shares can be likened to a trip to the supermarket.
The only difference is that rather than buying off the shelf, your transaction is carried out by a broker, a middleman authorised to buy and sell stock.
Instead of Tesco, Sainsbury and Asda, think London Stock Exchange, New York Stock Exchange, Hong Kong Stock Exchange and Tokyo Stock Exchange.
These world markets bring together people who want to buy shares and those who want to sell them.
In the UK, the market sub-divides, but we'll leave that until later.