These require endless streams of recruits. The recruits pay money to the recruiters, and enlist new recruits to give them money. Eventually, the number of recruits dries up. Be wary of businesses with the emphasis on recruiting, and not on products. This is a letter sent to a new recruit to forward on to others, placing their own name at the bottom of the list.
Recruits send money to the person at the top of the list.
Eventually, someone breaks the chain. Eventually, the recruits dry up, and the majority lose everything. This scheme is common in Eastern Europe, witness Romania in 1973 and Albania in 1997. Anything Nigerian- Be extremely leary of anything coming from Nigeria. There are many scams involving giving credit card or bank account numbers.