To expand on my previous post on Control Wards since I realized I don't have the greatest handle on the difference between camouflage and invisibility.

Champions with Invisibility and hence will not be revealed by a Control Ward
Akali
Kha'Zix
Shaco
Talon
Teemo
Vayne
Wukong


Ways to reveal an invisible unit

-Run near to a friendly turret
-Be a Burrowed Rek'Sai with the target moving (they won't show up to Tremor Sense if they are standing still)
-Hit them with a skill shot that does damage to make them shimmer briefly
-Activate an Oracle Lens or Sweeping Lens to outline the invisible unit.

Champions with Camouflage and hence will be revealed by a Control Ward

Evelynn
Rengar
Twitch

Works the same way as Invisibility except camouflaged enemies are revealed simply by walking within 700 units of an enemy champion, within 600 units of a Control Ward, or they can be revealed by champion abilities that grant unit-vision such as Caitlyn's Yordle Snap Trap or Lee Sin's first cast of Tempest.

You can kind of think of it this way. Invisibility tends to be relatively short duration and allows a champion to reposition and/or just avoid being focused for a little while, so it is harder to break in order to reveal the champion than camouflage.

Example - You are Kha'Zix and you get hit with CC a root or snare, you cast Void Assault to temporarily make yourself untargetable by anything other than AoE spells, so you hopefully don't get instantly swatted.

Camouflage tends to last a long time, giving the champion a way to bypass Stealth Wards but not Control Wards and it gives a warning indicator if you get too close to an enemy champion. Riot wants a mechanism to bypass stealth wards but not give a 100% free shot at sneaking up on an enemy champion (which is why Rengar is so obnoxious when played well/overtuned since he can leap onto you and burst you despite the warning indicator).

Twitch is obnoxious for the reason that he can open up on you with Spray and Pray from a range of 850 units (so outside of the radius that a Control Ward can reveal which is 700 units).

So of the three I consider Evelynn's usage of camouflage the most fair (even if it doesn't' feel like it when she is popping off XD) since she dosen't even get it until level 6.

Example - as Evelynn you are an assassin but you don't have any way to get over walls besides your ultimate Last Caress and since that is a large part of your burst you generally want to use that to make your escape AFTER unloading all your damage on your chosen victim. Your camouflage via Demon Shade allows you to bypass Stealth Wards, but for some measure of counter play you can't just walk up to an enemy champion if you want to get your full burst combo off because you will be revealed if you get within 700 units of your target. So you setup just out of range and use Allure on your chosen target. You then have to decide if you can kill them with just the slow or if you need to let the curse fully charge so that you can charm your target to bring them toward you.