What are SEO Backlinks? How many kinds do they Have? Please Help Needed.

I wanna ask what are backlinks?
Mainly at seoclerk people buy and sell that. Well i don't know them really. How many kinds do they have?

I know backlinks are essential to get high page rank.
But how many tipes of backlinks are? How to create each type?

What are these?

- Blog Comments
- Link Pyramids
- Link Wheel
- Lists
- SEnuke X
- Wiki Links

As i am interested to know more about them and even sell them as i am hard working man.