String

Is a sequence of bytes. The text is converted into those bytes usually by UTF-8 encoding.

String object are mutable.

Symbol

Symbol objects are immutable.

Same symbols, for example :book and :book, are in fact one object that is shared globally in all contexts for the duration of a program’s execution.

Thanks to that sharing symbols costs less memory than strings, but they cannot be collected by Garbage Collector once created.

Comparing symbols is quicker than comparing strings.