What information does ones P45 hold?

The form P45 is a form given to you when you leave an employment. The employer sends part 1 to his tax office to inform that you have left his employment. You keep part 1a for your records (especially if you want to claim a repayment), and you should give parts 2 & 3 to your new employer. He keeps part 2, and sends part 3 to his tax office to register your employment with him.
The form is designed to keep your employment history record complete, and enable you to get the full amount of tax allowances. It will show your pay and tax to date from the previous 6th April, your National Insurance number, your tax code number, the tax week/month you left your previous job, the date of leaving, and the reference number of your previous employer.
Re the link to the form. This is only the top copy. Not all that information goes through to the bottom copies.

it tell you how much income tax you have paid in this financial year.what you have earned and
what your NI number is and I think when your last employment ended

Here's a copy of the form itself.

http://www.hmrc.gov.uk/forms/p45.pdf