A self invested personal pension, or SIPP for short is a DIY pension, ideal for those who pretty much know what they are doing and feel confident to manage their own pension, choose investments that match their attitude to risk, and review them as required. A Stocks & Shares ISA is similar in that you can choose your own investments here too, but the main difference is the tax relief you get and the accessibility of your money. The returns from an ISA or a SIPP will depend on the success of your investment decisions.