HYIP stands for High Yield Investment Program. These can be both online and offline companies which offer a higher return on your investments than banks do. But due to the fact that offline HYIPs usually have very high investment minimums only online HYIPs are of interest to private investors. HYIP owners usually claim getting their above average returns on the stock market, FOREX, by placing bets on sports events or by participating in various online ventures. A part of the profits from these activities is shared among the investors according to the program's investment plans and the remaining amount is used to cover the hosting/maintenance fees and to pay the owners. There are two main types of online HYIPs - investment funds and autosurfs.
Investment funds work pretty much similar to banks. You invest some amount into one of the plans and get a daily/weekly/monthly interest credited to your account balance depending on the plan's rates. Autosurfs require you to visit some of their advertiser's websites to earn the daily ROI and you might not get paid if you don't. Both types of online HYIPs accept very low investments of $50 - $500 which is possible with the use of electronic currencies. The most important electronic currency in the world of HYIPs is Perfect Money. If you want to start investing you will have to open an account with them.
Unfortunately there are HYIPs that don't invest their members' money and work as so called Ponzis. They depend on new investors' money to pay the old ones. That is why it is very important to do your Due Diligence ( "DD" or "homework" ) on every program before you join. This means you have to study all the information available on the program such as hosting, script, feedback on forums and monitors and adjust your investments accordingly. If you do it right, you will be able to make some serious money with HYIPs. Good luck!