A typical atom consists of a nucleus that is surrounded by a cloud of electrons at a distance. The nucleus of an atom is usually made up of positively charged protons and neutrons that have no charge of their own. Electrons are negatively charged.
Most of the atoms in the universe follow this pattern, with protons and neutrons inside the nucleus and electrons swirling in various states outside of it. Some exceptions to the rule exist, however, such as hydrogen. The most common isotope of hydrogen, H1, has no neutrons at all. It consists of only a single proton in association with a single electron.
Other, more exotic exceptions are possible for the structure of an atom. Atoms made entirely of antimatter can be synthesized inside of large particle accelerators. These atoms consist of antiprotons surrounded by positrons in the place electrons would usually be found. Apart from their charges, atoms made from antimatter are structurally identical to atoms of ordinary matter and might behave in similar ways if they could be kept stable.
Perhaps the most extreme variation on the atomic model is the neutron star. Neutron stars are stellar remnants that have been so crushed by their own gravity that electrons within them have been forced to their surfaces. Essentially, a neutron star is one more configuration an atom can fall into.