In simple terms, a guarantee company is a not-for-profit company, and is the type most often formed by charitable organisations. A guarantee company does not have a share capital, and the members do not won the charity - they are decision makers for the company, but do not recieve any profits, and have no claim on the companies assets.
All income generated is used to cover operating costs and to achieve the objectives of the company.
Common guarantee companies often include clubs, student unions and charities.
Oxfam and nearly all other charities work under this business structure.