A "string" in Java (and most other programming languages) is simply a variable containing a series of alpha-numeric characters. Example: "I live at 123 Main street." Strings are distinguished from purely numeric variables like integers and floating point numbers in that you can't do math on them. Java also provides a string class that is used to manipulate strings.
Java String provides a lot of concepts that can be performed on a string such as compare, concat, equals, split, length, replace, compare To, intern, substring etc.