A share is a unit of ownership in a company.
When you buy a share you become a part owner, a shareholder in the company.
Stock exchange is a market on which shares are bought and sold or traded.
For a company shares to be traded on a stock exchange, they must generally be listed on that stock exchange.
All share prices are quoted with a two-way price or bid and offer spread which is the price that you can buy, also called the offer and the price you can sell. also called the bid, that particular share.
This price would update constantly throughout the market opening hours.
The width of the spread will be different for particular shares depending on the liquidity of the shares.
How easy it is to buy and sell shares and how much volume is being traded.