so many encryption functions in drupal7 who can tell us some ?

Noted there are many encryption functions in drupal7,
Can some one give us a link, or a little bit detailed explaination about
how and why one kind of encryption is applied.

for example

$hash = _password_crypt('sha512', $password, $stored_hash);

$hash = _password_crypt('md5', $password, $stored_hash);

$hash = hash($algo, $salt . $password, TRUE);

$output =  $setting . _password_base64_encode($hash, $len);

$itoa64 = _password_itoa64();

Thanks very much.