These are programs that are designed to leverage investments. Typical for investment programs is that there has to be an underlying mechanism that makes money. Common mechanisms are arbitrage bettings, buying and selling goods or commodities and loans.

Some prefer to see loans as a category of their own but from a simplistic point of view you can see them as an investment program. You put money in and hopefully more money comes out again.

A special category of investment programs are the so called HYIP programs. HYIP = High Yield Investment Program. These programs can be categorized as programs with very high returns. There isn't a specific return percentage carved in stone but basically anything between 1% and 500% percent a day qualifies as a HYIP program. In fact, I'd like to argue that anything from 25% annual and up  should be considered a HYIP program because compared to the interest the banks pay out on your savings account this is a very high return indeed.

By definition HYIP = High risk. As a rule of thumb you could say that the higher the returns the higher the risk is. So be careful when you invest in HYIP's. Only spend what you can afford to lose, get your principle back as soon as possible and spread your investments. A more detailed advise can be found under Strategy. 

Note that investment programs usually have some sort of affiliate program to attract more investors. It allows networkers to make extra income by referring others.