Introduction:
When playing for a team you may face the situation that a team has specific roles that players are assuming, mind you they are not permanent like in League of Legends. In CS:GO roles just specify what you are best at and serve as a general guide line for the team on who should pick up the awp, who flashes players in, who enters first..etc, but it does not cement you into that one role.

The official definition for lurking is picking rotators off, but a lurker does not necessarily mean only waiting for T/CT's to rotate, it can also be associated with flanking opponents (whic is also referred to as roaming, but for the sakes of simplicity I will refer to lurking as passive lurking and to roaming as active lurking).

Core Essence:
As a lurker your goal is to stay unpredictable, to hide in the corner where you would be least expected and to flank opponents. Everyone at one point or another has smacked their keyboard or swore out loud when caught off guard by a lurker "WHY WOULD YOU BE... WHO DOES THAT????". To be able to do this you need to understand how rotations work as well as to read the general game and the behaviour of players. Keep in mind that when lurking, in general, you put your team at risk since they are automatically down one player when entering a site.

What is a rotation? A rotation is when, for example, the terrorists decide to do an early push (on inferno) onto B through banana, but during the push one play gets popped in the process followed by the entire banana getting smoked off. Usually at this point someone calls for the team to fall back and to rotate into apartments or in general onto A. Same way as the CT's called a banana push and one or even two CT's could have potentially rotated from A site to B and back to A.

As a lurker you have to catch them (the CT's) off guard and pick one or two off. There are two ways you can, "lurk" flanking (Active Lurking) or the term that requires marshmallow: camping (Passive Lurking). An active lurk will usually mean a four-to-one split. Let us take as an example the map Inferno, let us say the scenario is that the team decides to push A after the car on banana gets smoked off and an awp is fired through the smoke. You decide to lurk this round and assume your position on banana.

You can assume one of the two (marked as orange stars) positions. Being positioned closer to the (grey filled circle) smoke can have a higher chance of catching any CT that tries to push through banana onto mid off guard, but it does not exclude the second hiding position either, it is a spot checked most of the time. Assuming that neither of the two CT's go onto mid you push through onto B around the same time the bomb is planted (a point at which you can assume, unless saving, that no-one is on B). Now you can sneak up behind the CT's that are in arch/library and bam, keyboard are broken, not really, but you can be sure that the guys you kill will be slightly annoyed! You can also alternatively stay around the ruins area and kill any CT that tries to save their weapon.

The other, more stationary, option that is refered to when speaking about lurking is passive lurking (camping) for rotators. We shall stick to Inferno for the time being. When passively lurking you are closer to your team which is a bonus, unlike when you are actively lurking where you might end up being on the otherside of the map. Your team decides to do an aggressive push/rush onto B site, a great position (that has worked time and time again for me) is rats (underpass). When sitting in rats you can hear the footsteps run over your loaction indicating you to get out and possibly earn 1500$. You should be wary of a potential second player, sometimes instead of one flanker there might be two, unless you are certain that there is only one player do not reveal yourself. If you keep lurking never assume the same position two rounds in a row since now there is a high probability that the flankers/rotators will check your old spot.

Conclusion:
Passive lurking can be tormenting, but patience (and accuracy) will bring reward and a possible round victory whilst active lurking is "more fun", but riskier. Keep in mind when lurking you have to have some game sense, you have to figure out how players in that particular lobby think, otherwise you might be lurking for no reason/inefficiently. Remember, you are lurking to catch rotating/flanking players off guard, timing and patience is the key and as always, you can and should apply these ideas when playing on othermaps.