In the natural universe there are four natural states of matter. The first three are gas, liquid and solid and generally very easy for us to understand as we interact with them regularly. In many ways they are very similar. The fourth is plasma, a form of matter that is in some ways exotic to us, but in the universe at large, it makes up 99 percent of the visible matter including stars, nebula and ball lightning.
Plasma is, at its most basic, ionized gas. This means it is made up of electrons and ions. These ions are positively charged matter of almost any type of matter, though typically it is lighter elements. Since electron bonding is how we classify matter and its reactions, this puts plasma outside of our normal understanding. It also makes plasma more unstable than the other forms of matter, because without specific environments, they will recombine and form into gas which can then turn into the other forms of matter. This environment is simply enough energy to separate the electrons from the molecules.
On earth, the most common naturally occurring forms of plasma are lightning, and more specifically ball lightning and the northern lights. Far more common, though, are those that humans have created. Fluorescent lights are one of the most common human uses of plasma. Less common, but still used, is the plasma torch, which can be used to cut steel more effectively than other methods.
One of the most important aspects of plasma is that they are able to conduct electricity and respond to electromagnetic fields. This is easy to understand when you recognize that electricity is made up electrons and plasma is the separation of electrons and molecules. It is also different from gas which generally does not conduct electricity. This makes plasma very useful in a great many applications, many of which are still being explored, such as fusion. It can also be safer in many cases because the gasses which are turned into plasma when exposed to electricity are inert in other situations.
A full understanding of plasma requires an strong understanding of electromagnetic theory and by including 99 percent of the visible universe plasma is one of the most diverse forms of matter, as well, ranging in temperature from the hottest matter in the universe to plasma crystals which have been created in labs that are near absolute zero. What these have in common is that the electromagnetic bond has been overcome creating matter that is very different from what we are used to in our everyday lives.