chemical bond
Think of two magnets that snap together and then resist being pulled apart. Atoms do something similar, but the glue is not magnetism in the everyday sense — it is the electric attraction between negatively charged electrons and positively charged nuclei. When two atoms come close and arranging their electrons in a shared way lowers the total energy, they stick together. That sticking is a chemical bond.
More precisely, a chemical bond is a lasting attraction between atoms (or ions) that holds them together in molecules, crystals, and other structures. It forms because the bonded arrangement has lower energy than the separate atoms, and you would have to supply energy to break it apart. The main families are covalent bonds (atoms share electrons), ionic bonds (one atom hands electrons to another and the resulting ions attract), and metallic bonds (positive ions sit in a shared sea of mobile electrons).
Why it matters: bonds decide almost everything about a substance — its shape, its strength, what it reacts with, whether it conducts electricity, and how much energy is released or absorbed when it changes. A useful caveat is that these bond types are idealized ends of a spectrum; most real bonds are a blend, partly sharing and partly transferring electrons rather than one pure kind.
In a water molecule, each hydrogen atom is held to the oxygen by a covalent bond — a shared pair of electrons. Breaking those bonds (splitting water into hydrogen and oxygen gas) takes a large input of energy, which is exactly why water is so stable and why electrolysis needs a battery to do it.
Stable substances are stable precisely because their bonds cost energy to break.
Bond and intermolecular force are not the same thing. A chemical bond holds atoms together inside a molecule; weaker intermolecular forces hold separate molecules near each other. Boiling water breaks the forces between molecules, not the bonds inside them — the steam is still made of intact water molecules.