The article explains the basic principles of income tax, focusing on the UK system. It discusses the year of assessment, sources of income, and the requirement for UK residents to pay tax on both UK and foreign income. Double taxation relief is provided for foreign income already taxed in its country of origin. Some types of money received are not considered taxable income according to judicial rulings. The Income and Corporation Taxes Act of 1970 does not define income but lists sources of income, as what is considered income can vary between individuals.