What are Inertial and non-inertial forces? What are inertial and non-inertial forces? I looked them up. 
One of the websites says that an inertial force is a force that can be observed/measured in an inertial frame. In this case, do we have a Force that we can't observe or measure in an inertial frame? Aren't forces frame independent? 
The website also says, "A non-inertial force is what is called a fictitious force that appears when you’re measuring things in a non-inertial frame of reference." 
Is that it? Non-inertial forces are basically pseudo forces that we use in non-inertial reference frames?
 A: I would not use these terms, because I think they are misleading.  A force is a force.  If a force is present in a system, then it will be present in any reference frame, although the way observers perceive it might be different between different observers.
Fictitious forces are, as their name implies, not real.  If you are standing in an accelerating train, then from your perspective, you will think that there is an invisible force that is pushing you towards the back of train.  But this force doesn't exist, the push back that you feel is purely due to the fact that you are in a non-inertial frame.
You can also have the opposite effect.  Take a free falling observer.  Then that observer is accelerated by a force towards the ground.  But from the perspective of the observer, he feels weightless, i.e. he doesn't feel the force of gravity acting on him.  The frame of the observer is non-inertial.
A: 
Inertial reference frame implies, if two co-ordinate systems are in a constant relative speed respective to each other, then their relative speed is zero. Let us assume co-ordinate_1 and co-ordinate_2 are in constant speed relative to each other. Their relative speed is zero. A force here will follow the newton's 2nd and 3rd law of motion properly. In other words, If you can achieve Newton's law of motion in a frame of reference they are Inertial reference frame. 
Non-inertial reference frame implies, co-ordinate_1 and co-ordinate_2 are not in the constant motion. They are accelerating with respect to each other. Suppose, co-ordinate_2 has an acceleration respect to co-ordinate_1. There will be a force working on the direction of the acceleration. Observer in co-ordinate_1 will know the reason of force. But observer in co-ordinate_2 will only observe the impact of the force. 
There is a good example. Suppose there is a bus turning a curve. There is a centripetal force working towards the direction of radius vector. The Centripetal force is responsible for the bus to complete the turn. What about the passengers? they will also feel the same force but the opposite direction of centripetal force. This is called the centrifugal force. We call it pseudo force or fictitious force. This centrifugal force working on the passengers in the bus is a non-inertial force. Because it is in the non-inertial frame of reference.      
