The Big Bang is a theory that suggests the universe came to be all at once, and is constantly expanding and moving outwards. The initial explosion is what cause the universe to continually expand. It is the most likely the best theory because there is a multitude of evidence that supports it, and it has not yet been dis-proven yet.
The Steady State theory states that the universe has been and will always be the same. The universe does not expand or shrink. It also states that the universe is at a constant temperature and does not have any background radiation. It says that new matter is being continually created from nothing, but in minuscule amounts. It also says that the concentration of matter will remain the same.
The Big Chill states that if the universe does not have enough matter in it, the gravity within it would be too weak to stop it from growing. The universe would eventually grow until all the stars burned out. We would then have no light or heat.
The Big Crunch is a theory that states the universe will expand to a maximum expansion, and then crunch back down out of existence and time will cease to exist. Dark energy will cause a turn around that overcomes the gravity pushing outward.