Well I understand an internet business is one that has no other outlets except the internet.
Amazon for example only sells over the internet.
I do not fully understand the links business but I believe you can make money on YOUR site if you provide a link to another site and someone clicks on it and goes to the other site.
So for example, YOU set up a web site, and provide a link to say Dell or Amazon.
If somebody who is on your site clicks on the Dell or Amazon link then you get some money from Dell or Amazon because it increases their traffic.
I am not sure it is a lot of money, but if hundreds of people do it then the money would mount up.