A sequence is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp, lists and arrays. Thus, an object of type list or of type array is also considered a sequence.
Arrays are further subdivided into strings, vectors, char-tables and
bool-vectors. Vectors can hold elements of any type, but string
elements must be characters, and bool-vector elements must be t
or nil. The characters in a string can have text properties like
characters in a buffer (see section Text Properties); vectors and
bool-vectors do not support text properties even when their elements
happen to be characters. Char-tables are like vectors except that they
are indexed by any valid character code.
Lists, strings and the other array types are different, but they have
important similarities. For example, all have a length l, and all
have elements which can be indexed from zero to l minus one.
Several functions, called sequence functions, accept any kind of
sequence. For example, the function elt can be used to extract
an element of a sequence, given its index. See section Sequences, Arrays, and Vectors.
It is generally impossible to read the same sequence twice, since
sequences are always created anew upon reading. If you read the read
syntax for a sequence twice, you get two sequences with equal contents.
There is one exception: the empty list () always stands for the
same object, nil.
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