An Instagram account custom audience allows you to target people who engaged with your Instagram business account with Facebook ads.
Lead form custom audiences allow you to create audiences of people who engaged with your Facebook lead ad forms to be used in ad targeting.
A saved audience allows a Facebook advertiser to save often-used targeting settings to easily use later.
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.