Sparrows explanation is very good. I'll expand regarding attack bonuses that apply to stealth. I'll also add the term crouch: the walking around your character does while still in sneak, which drains stamina.

If a skill or set mentions a bonus from stealth or sneak attack, it means essentially the same thing...that you need to be in crouch and hidden from all enemies for the bonus to apply.

For example, Archers Mind set: attacks from stealth increase crit damage by 10%. To get this, you need to be in sneak mode with the crosshairs eye closed (a sound also triggers as you go in and our of sneak). No enemies can see you. If you're playing pvp and bow ganking someone across a river from sneak, maybe you're standing next to a mud crab that's seeing you. The human player can't see you just because the crab does, but the crab is pulling you from stealth and will prevent the buff from proccing

In pve, attacks with stealth bonuses usually need to occur behind the target unless it's a range attack. For example, the stam blades surprise attack...huge bonus for an attack from stealth, but to stay stealth you need to sneak up behind the monster (usually).

Some sets and skills reference bonuses for attacks from crouch, and not stealth/sneak (vicecannon of venom set). These are rare, but my assumption is that the buff still applies even if you're seen. So I'm attacking someone across the river in crouch but the mud crab sees me...I think the bonus applies. Or maybe I launch four arrows at the other player. After the first, they can see me, my stealth is broken. But I'm still in crouch, so hits 2-4 can still prop the crouch bonus.