Gluon is an elementary or fundamental particle. It is the carrier particle for strong force, just like a photon is force carrier for electromagnetism, the W and Z bosons for the weak interaction or theorised graviton for the gravitational force. You could imagine gluons "gluing" quarks into protons and neutrons.
Force carriers are called gauge bosons. Like all bosons, gluon has integer spin and it equals 1. Theoretically, it has zero mass.
Gluon carries no electric charge, but it carries the colour charge of the strong force. This differentiates gluon as a gauge boson from photon, since photon carries the electromagnetism, but doesn't have electric charge. This means that gluon not only carries the strong interaction, but participates in it.
Colour charge is strong interaction's version of electric charge that you may be more familiar with. It is a property that doesn't manifest at distances larger than that of an atomic nucleus and it has nothing to do with colour as we perceive it. Term "colour" was chosen because there are three different charges that interact among themselves which is similar to three primary colours in the RGB model– red, green and blue. There are also three negative charges, anticolour charges named antired, antigreen and antiblue, bringing the total to six.
So, which colour charge does a gluon carry? Well, a single gluon carries a combination of a colour and anticolour charges, but we can't know exactly which one, only the probability for some particular quantity that we measure. To keep it very simple, this means there are eight different possible combinations of colour charges and thus eight different types of gluons.
Since gluons carry colour charge and participate in strong interactions, they are confined within hadrons. This is why the strong force between protons and neutrons is not carried by gluons, but by mesons. Because of this colour confinement, when we try to break apart a hadron at low energies, it is impossible to get free gluons.
There are two possible states where gluons could exist outside of hadrons. One possibility is a glueball - hadrons made entirely of gluons, but this state hasn’t yet been observed. The other one is quark-gluon plasma where under extreme pressures and temperatures quarks and hadrons become free particles. This state was confirmed in 2010 at the Large Hadron Collider.