When you're working you'll be sent forms about the tax you pay on your salary. Shortly after the end of each tax year, your company will send you a P60. This is a statement that shows the salary you have earned that is taxable and the tax you have paid for the tax year just ended.
If the company provides you with certain benefits, like a company car or pays your medical insurance premiums, you'll have to pay tax on those amounts. Each year, the company will send you a summary of the benefits you will have to pay tax on, called a P11D. The company will also give your tax office a copy of the information. Your tax code will take into account any taxable benefits you receive.
You will need these forms to help you complete your tax return or apply for tax credits.
When you leave employment you'll receive a P45. This shows the taxable income you have earned and the tax paid for the tax year to date. You'll need to give this to your new employer so that they know how much tax to take off you. If you don't give your P45 to your new employer, you might find yourself initially paying too much tax.