It's not encryption, it's a one-way hash.
There are a handful of different password hashes usually used for Linux system users' passwords, they're listed in the man page for crypt(3)
The first is the original crypt algorithm, that only supported 8 character passwords (among other flaws), and which you'll hopefully never see again.
The second is the MD5-based md5crypt ([a], [b]), marked with
$1$. It's considered dated mostly because it doesn't support changing the amount of iterations, i.e. the cost of the computation.
Currently more used are the SHA-256 and SHA-512 based hashes, sha256crypt and sha512crypt, which are similar in structure to md5crypt but support variable amounts of iteration. They're marked with
$6$ respectively. As @SEJPM quickly enough commented, sha512crypt (
$6$) is what at least Ubuntu and Debian currently use by default.
$6$, the characters after the identifier up to the next
$ are the salt, which is generated randomly when the password is changed.
The remaining option, not in mainline glibc is the Blowfish-based bcrypt, marked with
$2a$ (or another letter). Bcrypt also uses a salt.
For more information on password hashing, and as to why use salts and why slower is better, see
"How to securely hash passwords?" on security.SE.