WordPress Codex: is_single()

This Conditional Tag checks if any single post is being displayed. This is a boolean function, meaning it returns either TRUE or FALSE.

Usage

Parameters
$post
(mixed) (optional) Post ID, Post Title or Post Slug
Default: None

Return values
(boolean)
True on success, false on failure.

Examples
is_single();
// When any single Post page is being displayed.

is_single(17);
// When Post 17 (ID) is being displayed.
// Integer parameter also works

is_single('beef-stew');
// When the Post with post_name (slug) of "beef-stew"
// is being displayed.

More information: http://codex.wordpress.org/Function_Reference/is_single