Started hearing it since as a kid, right? In NCERT curriculum we start to dig on the topic, in class 9.
According to classical definitions in physics and general chemistry, matter is anything which occupies space by having volume and has mass. The objects which we see, tough and feel, consists of atoms and molecules which in turn has mass and occupy a specific space, turns out to be a matter.
 In everyday life, matter is seen in three different states, which in curriculum we call as, the states of matter. The solid, the liquid, and the gas, are the three basic types of matter whereas other states are also possible, which includes, plasma, Bose-Einstein condensate, Fermionic condensate and quark-gluon plasma.
Different types of matter can combine to form substances that may not resemble any of the original ingredients called the reactants, which as discussed in chemical reactions. For instance, hydrogen (a gaseous element) and oxygen (another gaseous element) combine to form water (a liquid compound at room temperature). A chemical reaction involves interactions or bonding between the electrons of the atoms, but does not affect the nuclei of the atoms. The nuclei come into action only in nuclear reactions, which is a more wider topic of fission and fusion, generally, the radioactivity.
In some conditions, matter is converted into energy by atomic reactions, also known as nuclear reactions. This type of reaction is fundamentally different from the chemical reaction because it involves changes in the nuclei of atoms. The most common example of an atomic or nuclear reaction is the nuclear fusion of hydrogen that occurs inside the sun. The immense pressure and temperature inside the sun, and inside other stars, forces atoms of hydrogen together to form atoms of helium. In this process, some of the mass is converted to energy according to the famous mass energy relation of Einstein,
E = mc 2
Coming back to the states of matter-
In Solids, the particles or molecules are tightly packed so that they don’t have enough space to move around. The electrons revolving around the nucleus are in constant motion which results in vibration in an extremely small scale. Yet the atoms are fixed in their position. As motion is restricted, the total kinetic energy of the particle is very less.
The solids possess a definite shape, definite mass and definite volume. Solids possess high density which again gives brief idea about how tightly the particles are packed.
In liquids, the particles are free to move a bit more than in solids, that is the atoms or molecules in a liquid are more loosely packed than in solids. This results in flow of molecules, thus the name fluids, give them an indefinite shape. This results in liquids acquiring the shape of the container in which it is kept.
Liquids possess lower density than that of solids and are difficult to compress. Thus liquids are also known as incompressible fluids.
In Gas, the entropy or the freedom of movement of particles is very high. The molecules are loosely packed and have a large space between them resulting in higher kinetic energy. It too acquires the shape of the container in which it is kept and it spreads out indefinitely in the confined space. The gas do not possess a fixed volume as it has a great deal of space between the particles and hence they are highly compressible and are called compressible fluids.
The fourth state of matter, the plasma is not a common state here on earth, but it is considered as the most common state of matter in the universe according to famous laboratories around the world. Stars, according to them are giant superheated balls of plasma. It consists of highly ionized or charged particles with soaringly high kinetic energy.
Bose Einstein condensate or BEC is created in 1995 by a group of scientists. Using a combination of magnets and lasers, scientists at Colorado JILA cooled down a rubidium sample within near absolute zero. At this low temperature its molecular motion nearly stopped and thus resulting in null kinetic energy. This resulted in energy transfer from one atom to another resulting in clumping up of atoms, just one super atom. Even light appeared to slow down as it passes through a BEC allowing study on wave particle paradox of light. BEC also exhibits properties of superfluid and are used to simulate conditions that might be similar to that which exist in a black hole.
The states of matter are interchangeable by several process or phenomena such as, freezing, melting, evaporation/boiling, sublimation and condensation.
Research has shown that there may be other states of matter that need further exploration, Which is left for another discussion… Stay tuned.