Taxation is differentiated from other forms of payment, such as market exchanges, in that taxation does not require consent and is not directly tied to any services rendered. The government compels taxation through an implicit or explicit threat of force. Taxation is legally different than extortion or a protection racket because the imposing institution is a government, not private actors.
Tax systems have varied considerably across jurisdictions and time. In most modern systems, taxation occurs on both physical assets, such as property, and specific events, such as a sales transaction. The formulation of tax policies is one of the most critical and contentious issues in modern politics.
International Taxation under DTAA.