OK, now what? Well now that you know which boards aren't straight, flat and with square edges you need to make them straight, flat and with square edges. If the Cup, Bows and Twist aren't too bad you may be able to start with the jointer. More than likely, if the boards are long, or very wide, you may have to rip or cross cut some into shorter or narrower pieces. Here's why.
Let's look at a bowed board. The illustration below shows you the flat, square cornered board you can expect to end up with (the cross hatched area) You'll get a long thin board if you want to keep the lenght. BUT if you cut the length in half you can get two thicker boards though they will be shorter. Either way, you'll use the JOINER to get one face flat - specifically the CONCAVE face flat since it involves removing less material and the board won't rock up and down while joining it. After you have one face flat you can plane the other face to parallel the joined flat face.
The same type of option is applicable with a crooked board only rather than joining the face and then planing the other face to parallel the first you need to rip the sides to get the edges of the board straight and parallel.(more on ripping later)
The same option can be used on a cupped board. Join it "concave face down" then plane the opposite face parallel OR rip the board in half, join the "concave face down" and plane to thickness. You'll end up with two narrower boards but they'll be thicker.
Now on to what to do with a twisted board. The example shows a board with a 20 degree twist and the same board cut in half lenght-wise to cut the twist angle in half or 10 degrees. As you can see, if you cut the length in half you end up with two shorter boards - BUT they're thicker.
Now that you know what you need to do it's time to talk about joining, planing, grain direction and tear out. It's a rare thing to find a board that is all straight grain and with the grain running parallel to the edge and face. So, for the stock you're most apt to work with, the orientation of the grain to the cutter head on the joiner and planer is important.
The two diagrams above illustrate why the grain direction is important. On the left image you can see that the cutter knife will cut the wood rather than catching on the edge of the grain and tearing the wood. On the left note that the knife, rather than cleanly cutting the wood, can catch the edge of the grain an tear out a chunk of wood rather than cutting it off cleanly.