Type-1 Antipersonnel grenade

The Covenant Type-1 Antipersonnel Grenade, also known as the Plasma Grenade or by fans as Sticky Grenade, is similar to the UNSC Frag Grenade in that it is a thrown anti-infantry and anti-vehicle weapon.

Function
The Plasma Grenade sticks to targets and is impossible to release. It has some kind of internal mechanism that allows it to distinguish between targets and inanimate objects. For example, it will stick to a soldier or vehicle, but not a tree or inorganic wall. It has a three-second fuse that is activated after it sticks to a target or otherwise comes to rest.

It should be noted that prolonged exposure to the radiation emitted by plasma grenades can cause a deviation in the neural electric pathways of the user--a disorder called Boren's Syndrome, which in one case renders them immune to infection by the Flood.

Advantages
Sticking an enemy with the plasma grenade kills them when it detonates. Even a fully shielded Spartan or Elite (with the exception of higher-ranking Elites like Ultras on higher difficulty levels) cannot withstand a stuck plasma grenade (it even kills if they have overshields). Also, the plasma grenade is slightly more powerful than the UNSC fragmentation grenade. If it goes off at the base of the target's feet (Does not have to be stuck) it results in an instant kill. This is probably due to the fact that plasma can melt through metal and would thus cause lethal damage even to a Spartan in MJOLNIR armor. This attribute is only featured in Halo 2 and in Halo 3. The one exception in Halo 2 is if a player has a fully loaded Overshield the grenade will heavily damage him and can be killed almost immediately after. In Halo 3, the Overshield is weaker than in Halo 2, so a stick is lethal in 3 regardless of shield status. Invulnerability, of course, is not affected.

Disadvantages
The plasma grenades bright blue glow and longer delay fuse allows enemies to easily avoid it. It can not be used around corners well, as it does not bounce, nor does it roll. Also, if a smart enemy is stuck when they are too close they can simply run toward you to catch you in the detonation. In Halo 2 and Halo 3, a primed plasma grenade in flight can be detonated mid-air by another explosion. Weapons capable of this include the Rocket Launcher, Brute Shot, Missile Pod, Frag Grenades, other Plasma Grenades. They can also be shot out of the air with certain weapons such as the M41 LAAG and the AIE-486H Heavy Machine Gun, though it is very difficult. It is also known to expose the thrower to deadly radiation, a case of this being Sergeant Avery Johnson, who had contracted Boren's Syndrome from a box of Covenant plasma grenades.

Theory
It has never been clearly stated how the Plasma Grenade adheres to objects. Here are a few theories on this subject. Note: the following is speculation and is not considered canon. The plasma grenade may stick by producing a thin layer of plasma which, when in contact with a target, fuses to the target. The reason the plasma grenade sticks to vehicles and characters may be due to an ability to sense heat. Since a "dead" wall doesn't emit heat, it will however stick to the "living" walls of a Flood Hive. Body heat and the heat from vehicles engines is most likely what causes it to stick.

Trivia

 * In Multiplayer, if you stick an enemy with the plasma grenade you are awarded with the Grenade Stick Medal.
 * On very rare occasions, a Grunt may try to throw a grenade and it will stick to his hand.
 * In Halo 2, when the Covenant use plasma grenades in battle, they will shout phrases as they throw them, such as "cleansing flame", "flare" and "holy light". This is similar to their belief of Halo.
 * If John-117 throws a plasma grenade, the Covenant will call it a "demon flare". However, when the Arbiter throws a plasma grenade, Elites and Grunts will still address it as a demon flare, even if the grenade is a frag. This may prove that all kinds of enemy grenades are demon flares. This may be as a kind of slang, with "demon" referring to the enemy, or enemy thrown grenades, and holy in the opposite manner.
 * Often times, when an NPC is stuck, they will have some type of reaction to being stuck.
 * Occasionally, if two plasma grenades are thrown in succession at the same spot; one will detonate, launching the other into the air. The airborne grenade will explode in mid-air, causing a "fireworks" effect. This was easier to perform in Halo: Combat Evolved.
 * When performing the "fireworks" effect in Halo: Combat Evolved, if the plasma grenade was originally on a place that should blow sand out, it will still blow sand out in the sky.
 * If you stick a charging Hunter with a plasma grenade the Hunter will instantly turn around, exposing its back, when the grenade explodes.
 * In Halo 3 a panicking Grunt may light 2 plasmas and, whilst holding on to them, run at you kamikaze, however the plasma grenades won't explode on a timer, they only explode when the Grunt is within range or if the Grunt is killed. Sometimes, if the IWHBYD skull is active, the Grunt will yell "Fuck it!".
 * In Halo 3 when you stick an enemy, the grenade will rapidly beep twice, then emit a high pitched whine.
 * In Halo 3 some enemies such as Scarabs and high ranking brutes cannot be stuck by these or spike grenades, the Brutes because of their Chieftan armor, and the Scarab for an as of yet unknown reason.
 * In Halo 3, if you look at the grenade bar in the top left corner, you will notice that there is a smilie on the plasma grenade icon, another example of Bungie's humor.
 * It is actually possible to stick an incoming spike grenade with a plasma grenade in Halo 3. (The spike grenade actually sticks to the plasma, not the other way around)
 * In Halo 3, if a plasma grenade is thrown at the wall of a Flood Hive it will stick.