This document is required when you are buying (or selling) existing shares in a company. It will set out the structure of the transaction, including the purchase price, conditions, warranties, etc.
Please do not use this if you are buying (or selling) the business or specific assets within the company. Parties to this document are usually the buyer and the owner / shareholders of the company (sellers). In return for the shares in the company, the buyer will usually pay the purchase price to the seller.