So what exactly is a share?
It is part ownership of a publicly-listed company. The company will usually also pay out some of its profits, known as a dividend.
But you may not always get a payout, particularly if the company wants to invest cash in the business, or has fallen on hard times.
British shares are traded on the London Stock Exchange. A record of their daily price movement can be found in the Financial Times.
If you want to track minute-by-minute changes, then you'll find them on internet sites such as ADVFN, or Yahoo Finance, albeit with a slight delay.
Buying and Selling Shares: You can always find live prices on ADVFN or Yahoo!
The market is sub-divided according to the size of a company.
The FTSE 100 is made up of Britain's 100 leading quoted companies - they are sometimes referred to as ' blue-chips' and are usually household names.
Currently, oil giants Shell and BP occupy the top two positions, while HSBC and GlaxoSmithKline are also among the upper echelons. Their worth is measured in billions of pounds.
Below the FTSE 100, is the FTSE 250 made up of medium-sized companies. Saying that, they are valued from around £250m to £2bn.
Moving on down, there's the small-caps (small capitalisation companies). The London Stock Exchange rather oddly refers to them as ex-350 companies, meaning that they rank below the top 350 firms in the market.
There is another wrinkle. The LSE runs a market for new, small companies called the Alternative Investment Market, or Aim. Its entry criteria aren't as arduous as a full listing, but investing on Aim is riskier.