What is the difference between an atom and an ion for a beginner?

An atom is the basic neutral unit of an element, with equal numbers of protons (positive) and electrons (negative), so the charges cancel and the overall charge is zero. An ion forms when an atom loses or gains electrons, so the proton and electron counts no longer match and the atom carries a charge. If an atom loses electrons it has more protons than electrons and becomes positive, called a cation, like sodium becoming Na+. If an atom gains electrons it has more electrons than protons and becomes negative, called an anion, like chlorine becoming Cl-. Importantly, the number of protons never changes, so it is still the same element, just charged now. So an ion is simply a charged atom, created only by a change in electrons.