a molecule is the smallest unit of a compound
The fact that oxygen has a slightly negative charge and the hydrogens have a slightly positive charge makes the moluecule slightly polar, because its slightly unbalanced.
Lewis dot structures are used to represent covalent bonds among atoms in a molecule. Each atom's symbol is surrounded by dots, representing valence electrons. Pairs of dots are used to represent covalent bonds, following the octet rule where each atom aims to have a full outer shell of electrons, typically 8 electrons.