In flying there are slips and skids. Each occurs when the plane is in uncoordinated flight, which means there is either too much or not enough rudder for the bank angle and turn. In a nutshell: a slip is not enough rudder, a skid is too much.

Slips and skids are NOT created equal. A slip is a standard safe maneuver. It is the proper way to land most small airplanes in a cross wind. It can also be used to lose altitude quickly without gaining excessive speed, for example to clear trees on short final then drop to the runway and land. Skids are more dangerous than slips, and there is no practical reason to do a skid, though they are commonly done in airshows as part of aerobatics.

Here's another way to visualize why skids and slips are different: In a slip, you are cross controlled - applying rudder & aileron in opposite directions. In a skid, you are NOT cross controlled - you are applying rudder in aileron in the same direction.

The practical difference between slips and skids is what happens if the plane stalls. If a plane stalls in coordinated flight, both wings stall at the same time - even if you're in a turn. This makes the stall simple and easy to recover. If a plane stalls uncoordinated (in a slip or a skid), one wing stalls before the other. This causes the plane to do interesting things. And as a pilot, you generally don't wan't the plane doing interesting things during a stall.

During a slip, the outside/high wing stalls first. In a skid, the inside/low wing stalls first.

When a wing stalls, it tends to drop. So if the high wing stalls, the plane levels, which is easy to recover. But if the low wing stalls, the plane falls inside the turn and wants to flip upside down, which is harder to recover.

A spin happens when the airplane is stalled, but one wing is more stalled than the other. Spins are not inherently dangerous, but they must be done in an airplane that is approved for spins and performed by a pilot proficient in them, with sufficient altitude to recover. You can enter a spin by stalling the plane while in an uncoordinated turn. Thus you can spin by slipping or skidding.

In a slipping spin, the high/outside wing stalls first, which rolls the plane out of the turn. At first this tends to level the plane. If you keep pushing the outside rudder pedal, the plane will continue rolling past level and flip "outside" or "over the top".

In a skidding spin, the low/inside wing stalls first, which rolls the plane into the turn, flipping it "inside" or "underneath".

It's generally easier to do a skidding spin and harder to recover if it happens suddenly. It's harder to get a plane to do a slipping spin, and it's almost self-recovering, since the roll it causes tends to return the plane to level flight.

Here's another way to think about which rudder pedal is safe vs. dangerous: think about which wing is high vs low. If you stomp the pedal on the same side as the high wing: that's a slip and it's usually safe. If you stomp the pedal on the low wing side: that's a skid and it's dangerous.

Now consider flying in a straight line - not in a turn - for example on final approach to land. Suppose you have a moderate to strong crosswind. You bank the airplane into the crosswind (lower the upwind wing). But you don't want the airplane to turn into the wind, so you apply opposite rudder. You are pressing the rudder on the same side as the high wing. Make sure you keep the nose down! This is a slip and it's safe. Usually, the ailerons have more authority than the rudder. That means if you stomp the rudder pedal to the floor, it will take less than full deflection of the opposite direction aileron to fly in a straight line.

This makes it easy to fly the maximum slip the airplane can do. Simply stomp the rudder pedal all the way to the floor and hold it there, and use however much opposite aileron you need to keep the plane going in a straight line. From that position, you can steer the plane with aileron, applying more or less to let the plane turn left or right. In this maximum slip it is important to keep the nose down and airspeed up! You do not want to let either wing stall.

And now you know why the maximum crosswind that a small airplane can land with, is determined by how much rudder authority it has.

In practical terms to take away: