Shakes, Checks and Splits
All 3 of these types of separations in the wood run lengthwise, but they are different in that shakes are a lengthwise separation of the wood along the grain between the rings of growth, checks are separations of the wood occurring across the rings of growth, and splits are separations that run through the wood to the opposite or adjoining surface.
These separations usually occur at the ends of the wood. The answer in most cases is to simply cut off the defective ends and use what's left. Make sure not to waste the cut-off ends. Oftentimes, narrow pieces can be salvaged for use.
Knots are naturally occurring places in the wood that are branch bases, either from a branch growing from the trunk or a smaller branch growing from a larger branch. These are embedded in the wood and can sit in the wood tightly or loosely.
Like other defects, if the knot is at the end of the wood and can be conveniently cut off, then that might be your solution. If the knot is embedded tightly in the wood, then it can be placed strategically out of sight. On the other hand, if your project is of the rustic variety, you might just leave it in and expose it prominently. If the knot is loose, it can be stabilized with an epoxy and placed strategically out of view, or embraced for its natural charm in a rustic project.