A Molecule is what happens when several different Atoms decide they like each other and get together to form a Chemical compound. Some atoms are snobbish and don't form bonds with other atoms easily, argon and neon are examples. Other atoms really like each other and form strong bonds.
Types of molecules
Oxygen and hydrogen
Oxygen and hydrogen get together to form water (H2O) and that's one example. Oxygen and hydrogen don't always get together with each other. Sometimes a hydrogen atom makes it with another hydrogen atom and forms a hydrogen molecule (H2), the first hydrogen atoms formed 12-13 Billion years ago, less than a Million years after the Big Bang. Oxygen atoms also sometimes like each other and 2 of them form a molecule (O2).
2 oxygen atoms are often attracted to a single Carbon atom and form Carbon dioxide, CO2.
There are very complex molecules, protein molecules for example sometimes form long chains with enormous numbers of different atoms. DNA is a very long and complex molecule too.