Atoms are typically found in a neutral state, which means they have an equal number of electrons and protons. Electrons are negatively charged particles while protons are positively charged.
Electrons exist in areas around the nucleus of an atom, which contains the protons and neutrons. Electrons are able to move around and can even be transferred to other atoms. Since atoms try to be as stable as possible, which involves having a full outer valence shell of electrons, atoms often prefer to exist as positive or negative ions, and will give away electrons or steal them in order to become one.