- gas, and
Many other states are known too
These includes things such as Bose–Einstein condensates and neutron-degenerate matter but these only occur in extreme situations such as ultra cold or ultra dense matter.
Other states, such as quark–gluon plasmas, are believed to be possible but remain theoretical for now. For a complete list of all exotic states of matter, see the list of states of matter.
Historically, the distinction is made based on qualitative differences in properties.
Matter in the solid state maintains a fixed volume and shape, with component particles (atoms, molecules or ions) close together and fixed into place.
Matter in the liquid state maintains a fixed volume, but has a variable shape that adapts to fit its container. Its particles are still close together but move freely.
Matter in the gaseous state has both variable volume and shape, adapting both to fit its container. Its particles are neither close together nor fixed in place.
Matter in the plasma state has variable volume and shape, but as well as neutral atoms, it contains a significant number of ions and electrons, both of which can move around freely. Plasma is the most common form of visible matter in the universe.
Commentary note: so what is the difference between plasma and a gas in terms of being in a container? well a plasma typically is affected by magnetic fields and charges due to it having plenty of charged particles. So a container with plasma in it, is almost always in an excited state so tends to respond to magentic fields and/or charges. For example placing a hand onto of a plasma ball causes the plasma stream to be attracted towards the hand.
Note that there is a difference between phase of matter and states of matter. Phase of matter is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform.