A building society is a financial institution owned by its members as a mutual organization, which offers banking and related financial services, especially savings and mortgage lending. They exist in the United Kingdom, Australia and New Zealand, and formerly in Ireland and several Commonwealth countries, including South Africa as mutual banks. They are similar to credit unions, but rather than promoting thrift and offering unsecured and business loans, the purpose of a building society is to provide home mortgages to members. Borrowers and depositors are society members, setting policy and appointing directors on a one-member, one-vote basis. Building societies often provide other retail banking services, such as current accounts, credit cards and personal loans. The term "building society" first arose in the 19th century in Great Britain from cooperative savings groups.

A financial institution, sometimes called a banking institution, is a business entity that provides service as an intermediary for different types of financial monetary transactions. Broadly speaking, there are three major types of financial institution:Depository institution – deposit-taking institution that accepts and manages deposits and makes loans, including bank, building society, credit union, trust company, and mortgage broker; Contractual institution – insurance company and pension fund Investment institution – investment bank, underwriter, and other different types of financial entities managing investments.