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Simply put, covalent bonds are the 'glue' that holds atoms together within a molecule. They are formed when atoms share pairs of electrons, allowing each atom to achieve a full outer shell, which is a state of stability according to the octet rule. Take water (H2O) as an example; it consists of hydrogen and oxygen atoms bonded together covalently, with the oxygen atom sharing its electrons with the hydrogen atoms.

Unlike ionic bonds, which are formed between oppositely charged ions, covalent bonds form between atoms that have similar electronegativities. This means that the atoms involved in a covalent bond typically pull on the shared electrons with similar strength, resulting in a reliable and stable bond. The strength of a covalent bond contributes to a substance's melting point, boiling point, and its overall stability.

Broadly speaking, a chemical compound is a substance formed from two or more different elements that are bonded together chemically. Each element within the compound loses its individual properties to give way to new properties. For instance, sodium is a reactive metal and chlorine is a dangerous gas, but when they form sodium chloride (table salt), the compound is completely safe for consumption.

To be clear, the key to a compound is not just the physical mix of elements, but the chemical bonding that merges them into a unit with unique characteristics. Compounds can be broken back down into their constituent elements, but it requires a chemical reaction, sometimes simple, often complex, to disrupt those stable bonds.

Network covalent substances are fascinating due to their distinct attributes. They are iconic for having extraordinarily high melting and boiling points. This is because every atom is bonded tightly to its neighbors with covalent bonds, demanding massive amounts of energy to break these connections. The robust structure of network covalent substances also means they are not just strong, but also extremely hard and rigid.

When it comes to electrical conductivity, these substances generally do not conduct electricity, lacking the free-moving electrons needed for electron flow. However, a few, like graphite, make for notable exceptions due to their unique structural attributes. Another commonality among these substances is their insolubility in water and most solvents; because their atoms are interlinked so firmly, there's little opportunity for solvents to penetrate and dissolve the substance.

Diamond is the poster child for network covalent substances. It is more than just a symbol of commitment; it's a masterclass in atomic architecture. Each carbon atom in a diamond is bonded to four others, creating an endlessly repeating tetrahedral pattern that extends throughout the entire structure. This uniform and unyielding lattice awards diamond its hardness, which is unparalleled in the natural world.

Beyond its durability and sparkling allure, diamond's network covalent structure imparts high thermal conductivity while also making it an electrical insulator. Such a juxtaposition of properties makes diamonds not only a gemstone but also a material of great interest for industrial applications, including cutting and heat dissipation in electronics.