A saved audience allows a Facebook advertiser to save often-used targeting settings to easily use later.
Split testing (or A/B testing) allows you to test changes in Facebook variables like ad creative, audience, or placement to determine which approach performs best.
You can create an audience of people who viewed your video on Facebook or Instagram and target or exclude this audience in ads.
A website custom audience matches people who visit your website with people on Facebook. You can then create ads to show to that audience.
Website custom audiences from your events allow you to target people based on the specific actions and conversions performed on your website.