What password requirements exist for new users? My site contains a front-end registration form. New users will be automatically activated, and can begin using their account immediately.

What password requirements exist?

I vaguely remember a 6-character minimum... Is that accurate, and are there any other requirements to be aware of?
Craft 2:

By default Craft enforces a 6 character minimum as the only password requirement, but as of Craft 2.3, a plugin can now listen for the users.onBeforeSetPassword event to enforce any password requirements it wants for newly set/changed passwords and set $performAction = false to cancel the action with any validation errors returned on the UserModel.

Craft 3:

This has changed quite a bit for Craft 3... here is how you would pull it off:

use craft\elements\User;
use yii\base\Event;
use yii\base\ModelEvent;

Event::on(User::class, User::EVENT_BEFORE_VALIDATE, function(ModelEvent $event) {
    /** @var User $user */
    $user = $event-&gt;sender;

    if ($user-&gt;newPassword !== null) {
        $validates = $this-&gt;validateNewPassword($user-&gt;newPassword);

        if (!$validates) {
            $user-&gt;addError('newPassword', Craft::t('plugin-handle', 'Invalid password'));
            $event-&gt;isValid = false;
        }
    }
});