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A Share purchase agreement (SPA) is an agreement setting out the terms and conditions relating to the sale and purchase of shares in a company. (Rocket Lawyer)

Back before we had electronic computers and the Internet, keeping track of shares was a lot of work. If a person had 1,000 shares then you might have to move around 1,000 printed share certificates. Therefore the practice developed of converting a person's shares in a company into stock. Stock representing 1,000 shares could be documented as a single item instead of 1,000 items. (Of course, you could still sell one 1,000th of that stock if you wanted to.) (StackExchange)

This means that a shareholder can lose what they paid for their shares in a company but they are not liable for the debts of the company beyond that. (BBC News)