Virss

[Marcello]

Originally Posted by Wdll

What exactly are they?

It is a game abstraction which covers real world systems like this: http://fofanov.armor.kiev.ua/Tanks/EQP/shtora.html

Actual usefulness in real life may vary; I would suspect that for example the one in the link would work well against older TOWs, but will do very little against a Javelin. So you can see why not everyone would bother with them in real life.
The russians in particular pioneered jammers and active defenses, probably as a part of an effort to contain tanks weights and reducing the strain on the logistical system.