Friction is an important force that slows down or stops things from moving when they touch each other. There are four main types of friction, and each one works differently:

1. Static Friction: This is the force that keeps an object still until someone pushes it. You have to overcome this force to get something moving. The maximum amount of static friction can be calculated using this formula:
   $F_s \leq \mu_s N$
   Here, $\mu_s$ is a number that tells us how sticky the surfaces are (usually between 0.3 and 0.6 for common materials), and $N$ stands for the normal force, which is how hard the surfaces are pressing together.

2. Kinetic Friction: This is the type of friction that happens when an object is already moving. Kinetic friction is usually less than static friction. It can be calculated with this formula:
   $F_k = \mu_k N$
   In this case, $\mu_k$ is a number that usually ranges from 0.1 to 0.5 for everyday materials.

3. Rolling Friction: This happens when something rolls over a surface, like a wheel. The force of rolling friction is much weaker than sliding friction, about 1% of the force of kinetic friction.

4. Fluid Friction: This type of friction occurs when objects move through a liquid or gas. How strong this resistance is depends on what kind of fluid it is and how it flows. It’s often described using something called drag coefficients.

Knowing about these types of friction and their values is really important. It helps us understand how things move, design better systems, and keep everything safe in engineering.