What is the difference between 'particles', 'molecules' and 'chemical species'?
A chemical species consists of chemically identical atoms, molecules or ions (more generally molecular entities) and is identified as a single species in a spectroscopic experiment. The latter part of the definition is important, because, for example, a compound which is identified as a single chemical species by NMR spectroscopy can still contain different enantiomers (that are separate molecular entities) which cannot be distinguished by an achiral spectroscopic method.

A substance is matter which has a specific and distinguishable chemical composition and specific properties. From this follows that every pure element and chemical compound is a substance. For example, pure $\ce{Fe}$ and $\ce{CH4}$ are substances, while an aqueous solution of $\ce{NaCl}$ and gasoline are not. The latter two are a mixture of two ($\ce{NaCl}$ and $\ce{H2O}$) or more different substances and have therefore properties which depend on the composition.

Particles can be defined as microscopic constituents of matter, like atoms or nuclei. The term can also refer to even smaller constituents like electrons, protons, or neutrons, which are called elementary particles. It is also used in the definition of an atom, the smallest particle which is characteristic for a chemical element. Atoms with the same number of protons $Z$ (and equal number of electrons) represent the same chemical element. Isotopes are atoms of the same chemical element which have different mass numbers, i.e. the same number of protons $Z$, but different numbers of neutrons in the nucleus. In the three isotopes of hydrogen, for example, the nuclei contain different number of neutrons (0, 1 and 2).

A salt is a chemical compound which consists of cations and anions. It should be distinguished from a zwitterionic compound, which contains charged but overall neutral molecules as building blocks. An example for the latter is trimethylamine N-oxide.