It's not just the self employed who need to complete Self Assessment returns.
Income Tax is generally taken from your wages or pension income before it reaches you. But some income isn't taxed at source, like profits from selling shares or property.
Self Assessment is how HMRC collect tax from people and businesses with these types of income.
If you've received income which hasn't been taxed, hasn't been taxed correctly or has exceeded tax-free allowances, then it's likely you will need to complete a tax return.
Others, such as Company Directors, High Earners and High Earners claiming Child Benefit, also need to submit Self Assessments to HMRC.