Most framing wood used is pine, spruce or fir. Each wood has its own strength characteristics and must be properly selected when deciding what to use and where to use it. While standard pine/fir are fine for walls, they may not be adequate for a large floor or ceiling area. You may need to select a stronger material. There are lots of resources on line to help with this process such as southernpine.com which list all the load tables for pine. AWC has a calculator that has load and span information. Just make sure and don’t input some bizarre wood when using it.
Wood grades are another concern when understanding framing. Most framing uses #2 common.
See the NHLA website.
The best grade is FAS and it must be 83% clear or clearer on the poorest side. Clearness is measured in large rectangular areas called cuttings.
The next grade is No. 1 Common, which must be 67% clear on the poor side, etc.
Next is No. 2 Common, which is 50% clear. Then No3A Common, which is 33% clear. Then No. 3B, which is 25% sound.
There is also a grade called Select (which includes another popular grade called FAS 1-Face). Selects are FAS on the good side and No. 1 Common on the poor side.