Salt pork is basically what the name implies, salt cured pork. It can be made from the pork side, pork belly, or fatback. Depending upon where it is made from it can be lean, streaky, or completely fatty.
It is made from the same cut as bacon, so it looks like a bacon slab. The difference from bacon though is it is very salty, and is not smoked or bacon cured. It is used quite extensively for Boston baked beans, and others use if for cooking with vegetable and greens.
Streaks of lean is the name of a leaner version of salt pork of fatback. It has a lot more meat in it than other salt pork.