An identifier is a name that you assign to a data object. It can be a word, number, letter, symbol, or any combination of those. Ideally, the identifier is unique within your identifier system. A "persistent identifier" is an identifier that is available and managed over time; it will not change if the item is moved or renamed. This means that an item can be reliably referenced for future access by humans and software. Services can provide you with unique, persistent identifiers that you can apply to your data object and manage it over time.