SIPP stands for Self Invested Personal Pension so you might like to think of it as a Do It Yourself sort of pension. In reality, however, it's likely that you'll use the services of an advisor to help fill the SIPP up with the investments that make sense for you. Like all pension pots money going in gets topped up with a tax credit so your pension pot is a very effective way of saving for your retirement. When it's time to retire you cash in your SIPP and buy an annuity which is used to pay you your Pension.
SIPPs provide a more flexible method of building a Pension than "off the shelf" pension schemes as you can put a wide range of existing investments into your SIPP. For instance Commercial Property can be transferred into a SIPP - though not Buy To Let properties. The Buy To Let option was widely advertised but this was scrapped by the Government as was the notion of investing in fine wines. As you would expect, if you use an advisor there are costs for setting up a SIPP and managing it. There are set up, transaction and annual charges that have to be borne - though these costs have been coming down as more SIPP providers enter the market.