The word Hadrons refers to the large or massive in Greek. The advanced research in the field of physics and particularly sub-atomic particles has revealed the existence of elementary particles. Atomic nuclei is made from protons and neutrons, they are made from quarks, anti-quarks, gluons and are subject to the strong force. They are often called hadrons.
Hadrons (quark composites) are split into two main groups:
(2) Mesons -- composition of 2 quarks i.e. one quark and one antiquarks. Mesons are hadrons made from an even number of quark-antiquark constituents. The basic structure of most meson is one quark and one antiquark.
One example of a meson is a pion ( +), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion ( -) is made of a down quark and an up antiquark. Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K-) meson lives much longer than most mesons, which is why it was called "strange" and gave this name to the strange quark, one of its components. 
Elementary particles can be split into two main groups: fermions and bosons. Fermions are particles with half integer spin like quarks and leptons. Bosons are particles with integer spin like the gauge bosons (photons, W, Z, gluons etc). All composite hadrons are colorless, the colors of the constituent quark colors “canceling” each other out. Thus, baryons are composed of three quarks of different colors (since red, green, and blue together cancel) and mesons are composed of a colored quark and antiquark (represented by the bar overlying the symbol) of the corresponding “anticolor”(represented by the alternate stripe pattern).
The strong interaction acts on Hadrons (baryons and mesons) and holds them together. Nucleons have baryon number +1 and mesons have baryon number 0. Antiparticles have baryon number -1 (nucleons) and 0 (mesons).Charge, baryon number and strangeness are conserved in the strong interaction.
Difference between Leptons & Hadrons
Since the discovery of elementary particles is still relatively new, the majority of non-physicists are not as familiar with leptons and quarks, including hadrons, as they are with the sub-atomic particles, which makes it even harder for them to differentiate between the two categories of elementary particles. In order to understand the difference between leptons and hadrons, it is important to keep the building blocks of hadrons, i.e. quarks, in mind, as they play a key role in setting the two categories apart. One of the biggest differences between leptons and hadrons lies in their structure. While the lepton does not have a known internal structure and exists as individual particles, hadrons are made up of many quarks that combine with each other through strong nuclear interaction. They also have a known internal structure.
Another thing that sets leptons apart from hadrons is their size. The former are relatively smaller than the latter. These two types of elementary particles differ from each other on account of charge as well. Leptons typically have a -1 charge, whereas the building blocks of hadrons, quarks, have fractional charges. Another important thing to note here is that while all leptons are negatively charged, they have different masses. In the case of hadrons, however, there are anti-quarks that have the same mass but different charge.
The interaction among leptons and quarks also serves as a differentiating factor. The former interact through electromagnetic and weak force, while the latter interact through strong interactions. Source: 
Main Source: OpenStax College, College Physics. http://cnx.org/content/col11406/latest/ (ISBN-13 978-1-938168-00-0)