Strings are sequences of characters (like hello). Each character is an 8-bit value from the entire 256 character set (there's nothing special about the NUL character as in some languages).
The shortest possible string has no characters. The longest string fills all of your available memory (although you wouldn't be able to do much with that). This is in accordance with the principle of "no built-in limits" that Perl follows at every opportunity. Typical strings are printable sequences of letters and digits and punctuation in the ASCII 32 to ASCII 126 range. However, the ability to have any character from 0 to 255 in a string means you can create, scan, and manipulate raw binary data as strings—something with which most other utilities would have great difficulty. (For example, you can patch your operating system by reading it into a Perl string, making the change, and writing the result back out.)
Like numbers, strings have a literal representation (the way you represent the string in a Perl program). Literal strings come in two different flavors: single-quoted strings and double-quoted strings. Another form that looks rather like these two is the back-quoted string (`like this`). This isn't so much a literal string as a way to run external commands and get back their output. This is covered in Chapter 14.