Your personal tax code is the indicator of how much you can earn in a tax-year before you are liable for income tax i.e. your tax allowance.
Assuming you have the 'usual' code for this tax-year (2007-8) of 522L, this signifies that if you earn below �5225 (this figure is arrived at by tacking a 5 to the end of the 3 figures of the code) you are not liable for income tax. Any tax you may have paid whilst earning up to this amount can be claimed back.
If you earn more than your allowance, the amount over the allowance is taxed at the applicable rate e.g. earnings of �7225 would attract taxation of the �2000 over the �5225 allowance.
The amount of tax you should pay and the amount you do pay through the PAYE system (especially via Emergency Tax) do not always match up. Irrespective of how much you earned, if you have paid too much income tax you can claim back the overpayment at the end of the tax year, however the trick is calculating how much tax you should have paid.
Sites such as this are useful for that calculation...
http://www.listentotaxman.com/
Asking your employer for form P38S when you start your employment prevents paying income tax from the outset.