Sticky bundle of joy
The plasma grenade has the unique and fiendish ability of sticking to enemies or enemy hardware, except that it bounces off weapons, Jackal shields and stationary shields. Making it stick is called 'tagging'. This is both a valuable combat skill and a source of much comedy, thanks to the amusing reactions of tagged enemies. It has a delayed fuse lasting about 2.4 seconds on PAL Xbox or about 2 seconds on NTSC/PC, once it comes to rest or sticks. It doesn't bounce like a frag grenade; it seems soft and bounces like a bean-bag when it hits something it doesn't stick to. See Grenades for further advice.
In rare circumstances a plasma grenade may explode on hitting a surface, skipping the normal delay (e.g. see BCM55 at 4.07). I assume this is a glitch in the game, but it's perhaps more fun to instead think of it as being a faulty fuse or something!