The helm and crest are decorations placed atop the shield. The helm was placed atop the shield, a wreath was optionally above that, and a crest was at the very top.
Depending on the origin of the arms (France, Germany, United Kingdom, etc.) the color of the helm or the number of bars of the visor might indicate rank.
The wreath is actually called the “torse.” It is a twisted roll of fabric worn atop the helmet used to keep the mantling in place. It is typically made of two colors, usually the same colors used in the mantling.
In English heraldry, the crest is often a form of animal such as a lion but it may also be hands or arms holding weapons. Another common crest is bird’s wings. Other countries such as Germany makes the crest from a tall hat with a fan of plumes in alternating colors, or a pair of curving horns.