The idea is to buy things many years Before you retire which will increase in value, hopefully at a higher rate than inflation. Real estate can be a good investment, but there are taxes and upkeep costs. Stocks are a safe long term investment, but not so good in the short run. Mutual funds are just a group of people that pool their money and buy a variety of stocks to spread the risk. An IRA or 401k plan is good because it allows you to earn interest without paying income taxes, so more of your money gets invested. The key to a good plan is to start early, even if it's just a small amount each month it will grow.
I agree with drakk about buying real estate if you think you would like to be a Landlord, real estate is selling at an all time low . But if you don't think you have the time to devote to that investing in a selection of mutual funds will spread your risk over several different types of industries and sizes of companies. I would talk to a broker with one of the better known firms, one that I know of that will help a small investor get started is Ameritrade, but there are many others. Talk to several before you decide, it's important that you feel comfortable and trust the financial advisor you choose.