What is the difference a coat of arms and a crest?

What is the difference a coat of arms and a crest?

Peter O'Donoghue (Bluemantle Pursuivant ) gives expert video advice on: What is Heraldry?; What is a pedigree?; How do I begin to research my pedigree? and more...

The term coat of arms is usually used to refer to what's on the shield. Whereas a crest specifically refers to what's on top of the helmet. It's a popular misconception that the word crest denotes the whole heraldic achievement, the whole display or that you can use the word crest to refer to what's on the shield. But actually, if you use the term crest, it can refer only to what's on top of the helmet rather like a cochral.