Visual: Best looking by Korean standards
Center: The one who mostly gets a lot of center time
Face: The person you think of when you mention the group
Visual can have multiple, because there are multiple visual positions. Most of the time, the Face and the Center are the same people (and are also visuals). Like Yoona from SNSD, Yiren from Everglow. YooA from Oh My Girl.
Some groups don't actually have a center, but still have a Face Of The Group. Like for Day6 they don't have a Center, but Jae is the Face Of The Group