They're mostly pyramid schemes that you should stay far, far away from.
Can you elaborate please?
A pyramid scheme is basically getting 10 people to pay you money to enter into a business. And then the only way they'll make money is by getting 10 people to pay *them* to enter the business. And it all collapses when they reach saturation point and nobody is willing to pay them anymore...
So, take a case example of one of the more popular schemes of today: My Shopping Genie.
The scheme works on a modified version of pyramid scheme called "MLM" which is multi-level marketing. You pay $200 to be allowed to distribute the application. You then pay $30 per month extra for this "privilege". You get a few dollars every time somebody installs the application. And more when somebody signs on to become a distributor. They in turn pay the same amount to get other people to do the same. So on, so on and so forth until there are no more people to suck into the scam and it eventually crumbles in on itself spectacularly.
Most online "work from home" schemes are "PAY ME MONEY! AND I'LL GIVE YOU THE INFORMATION TO MAKE BUNDLES OF YOUR OWN MONEY!" And then sadly, the information you receive from them is to tell other people to pay you money... So you can tell them to get other people to pay them money...
That's pretty much it.