Not everyone automatically receives a tax return from the tax office. If your income just consists of a basic salary then you probably won't have to complete a tax return. However, if you have P11D benefits or income from savings and investments or property, then you'll need to complete a tax return detailing all your other income as part of self assessment.
You must tell HMRC if you receive taxable income in addition to any income you pay tax on through PAYE as you may have to complete a tax return form.
If you file your return online, the tax calculation is done for you automatically. If you send in a paper return, HMRC will calculate the tax for you or you can choose to do the calculation yourself. If you have not paid enough tax this can be collected through an adjustment to your tax code the following tax year. If you have paid too much tax then the amount can be refunded straight into your bank account.