The legs and top look like they are made out of nominal 2" framing lumber--2x4's or 2x6's, actually about 1 1/2" thick. This is rarely pine but rather a spruce/fir mix. This wood is processed for a structural rather than aesthetic end use. The kiln drying process is rapid to keep cost down and can result in fine cracks or fractures that run all through the wood. They may run at an angle to the surface, so they can appear and disappear as you look at it from different angles. They may open and close with changes in moisture content. The wood is stringy compared to pine, so the surface can be torn up by planing. The torn fiber in the bottom picture can go quite deep. It looks like someone worked this a bit with a hand plane, maybe to flush up the joint.
If the lower shelves are thinner, they are more likely to be a hard pine.
Don't know what the construction is, but it looks like a sturdy and nicely proportioned piece. What do you have in mind for a finish?