The MP5 is a moderately powerful submachine gun available to both teams. Its only major disadvantage is its low damage. It has one of the fastest rate of fires out of the SMGs, second to P90, and is the most accurate SMG along with the UMP45. Just like in real life, its 9mm bullets are stopped rather easily by kevlar. The MP5 is very attractively priced at $1500. For these reasons, the MP5 is very common in early rounds but is usually replaced by a rifle of choice in later rounds. Players on the losing side can afford the MP5 and kevlar to retain a competitive edge. The MP5 is one of the most iconic and most used weapons in Counter-Strike along with the M4A1, AK-47, AWSM and Desert Eagle.
Aim for the head or the chest and spray at close range. The most it can take to kill is 2 shots to the head of an armored enemy.
At medium range, aim for the stomach and spray, or burst for the head while crouching. This will hopefully result a headshot.
Crouch and shoot one shot at a time at long range. However, retreating may be a better idea unless you are up against another SMG user.
Use the fast rate of fire to your advantage by spraying.
Strafe continuously. An MP5 user is quicker and more accurate while strafing than a rifleman.
The reload could take some time, so make sure you reload regularly.
If you do score a headshot but the target is not eliminated, aim the MP5 at the lower sections of the opponent and keep firing or switch to a more powerful pistol, such as the Desert Eagle, to finish him off.
Use other powerful short-range weaponry like the P90 or a shotgun, or weapon with higher rate of fire at close. However, this will be slightly more difficult than the other options, especially if your weapon is a pump shotgun.
It seems that the MP5 was originally supposed to be usable in the beta stages of Global Offensive. In fact, at one point during the beta, it was possible to install the weapon as an addon and the MP5 could be used (see external links for additional information). However, in later updates, the MP5 eventually became unusable.