What is the difference between chest voice and head voice?
They are two registers, two ways the vocal folds vibrate. In chest voice the folds are thicker and vibrate along more of their length, which gives the full, speech-like sound of your lower notes; in head voice they are thinner and stretched, which gives a lighter sound higher up. Most untrained voices flip between them with a noticeable crack somewhere in the middle. Practice works on making that change smooth, so you can blend the two, often called mixed voice, instead of shouting the chest sound higher.