In my legal career I never had to consider what a bailment is exactly.
As I understand it from having read about it years ago, a bailment arises when something (it has to be a chattel) is left by one person with another for some purpose, usually to look after it or repair it, but not as security or so that the person it is left with has any rights in it or to use it, although they may have the right (called a lien) to keep the thing until some payment associated with it (such as a repair or storage bill) is made.