Everyone under the age of 65 has a personal allowance (£7,475 for the 2011/12 tax year) which is the amount they can earn before having to pay income tax.
Unfortunately employers will often automatically deduct tax from your wages. The problem is that HMRC may provide you with a tax code on the assumption that your earnings will continue over the full tax year, which they obviously won't. Accordingly they spread your personal allowance in the same way which means that you could end up paying tax when you shouldn't.
If you have earned under £7,475 then you can make a claim to HMRC for repayment of any overpaid tax back by using a P50 form.
But in future, when working a summer job, this problem can be avoided by speaking to your employer and completing a P38 form.