SIPPs or Self Invested Personal Pensions are simply an upmarket version of a personal pension plan. The only real difference is the wider range of investment options they offer.
The amount of pension you will get at retirement from a SIPP will depend on how much you invest, the growth of your investments, how much is deducted in charges and annuity rates (if you decide to convert your fund into an annuity at age 75).
As with other types of pensions, you will receive income tax relief on your contributions and the investments in your SIPP will grow virtually tax free. You can take a tax free lump sum, plus an income from your SIPP between the ages of 50 and 75, although from April 2010 the minimum age at which you can take retirement benefits increases to 55.