A character is a Lisp object which represents a single character of text. In Emacs Lisp, characters are simply integers; whether an integer is a character or not is determined only by how it is used. See Character Codes, for details about character representation in Emacs.
A string is a fixed sequence of characters. It is a type of sequence called a array, meaning that its length is fixed and cannot be altered once it is created (see Sequences, Arrays, and Vectors). Unlike in C, Emacs Lisp strings are not terminated by a distinguished character code.
Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions documented
in Sequences, Arrays, and Vectors. For example, you can access
individual characters in a string using the function
(see Functions that Operate on Arrays).
There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. For most Lisp programming, you don’t need to be concerned with these two representations. See Text Representations, for details.
Sometimes key sequences are represented as unibyte strings. When a unibyte string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See Character Type, for more information about keyboard input characters.
Strings are useful for holding regular expressions. You can also
match regular expressions against strings with
(see Regular Expression Searching). The functions
(see Simple Match Data Access) and
replace-match (see Replacing the Text that Matched) are useful for decomposing and modifying strings after
matching regular expressions against them.
Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See Text Properties. All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied.
See Text, for information about functions that display strings or
copy them into buffers. See Character Type, and String Type,
for information about the syntax of characters and strings.
See Non-ASCII Characters, for functions to convert between text
representations and to encode and decode character codes.
Also, note that
length should not be used for computing
the width of a string on display; use
string-width (see Size of Displayed Text) instead.