This document is by no means complete, and should by no means be refered to as an absolute authority on the gameplay of Doom.
In Doom, there are several 'bugs' in regular movement that, over time, became useful tactics that can provide an advantage over other players. See The Doom Wiki for completeness.
This is done by moving forward and strafing left or right at the same time, allowing you to move faster than you would normally (when you're only moving forth or backwards). This is a basic movement which, once you've played the game often, becomes a natural and effortless maneuver that you don't pay much conscious attention to.
SR50 stands for "straferun 50," and is basically a variant of straferunning that allows you to move even faster than normal straferunning. This maneuver is accomplished by simultaneously mouse-strafing and straferunning. To SR50, you move your mouse in the same direction as your straferun. While in SR50, you can not turn around due to mouse-strafing, so you sacrifice aim for mobility. This is a more advanced maneuver and is harder to control, but it is useful at times, especially for reaching a distant point faster or dodging an enemy's fire.
Although its use is limited, backward movement is actually faster than moving forward. Straferunning backwards is thereupon proportionally faster.
If you straferun at a certain angle against a straight wall while moving from north to south or from east to west, you actually gain even more speed than SR50. There is a server setting that enables wallrunning in all cardinal directions, but, by default, wallruns are only achieved in the two said directions -- from north to south, and from east to west.
Weapons in Doom are differently balanced than in most other games.
This is the most used weapon in Doom multiplayer. The combat shotgun, commonly referred to as the "super shotgun," or abbreviated as the "SSG," has historically become a highly abundant weapon on any given multiplayer map. Because of the powerful blows it can deliver, a well-placed shot can kill an opponent in one hit. However, it suffers from a major drawback; it takes considerable time to reload, forcing one to wait and thus allowing other, ready players to take advantage of this. Furthermore, it becomes inaccurate past medium range due to its large spread, limiting its efficiency to close combat.
The BFG9000 fires a large, green, and slow-moving projectile that can easily gib an enemy via a direct hit. After impact, a "cone" of invisible tracers is emitted from the position of the player towards the direction the player was facing after firing the BFG, regardless of the player's new position and orientation. The BFG9000's use can become highly complicated due to the many advantageous uses of the said tracers and the difficulty that can be experienced in dodging the tracers. This weapon is very often criticized for being overpowered and unfair by some, while others argue that this weapon is merely a necessary and original element of the game.
For a fully detailed explanation of how the BFG works, please read: The BFG FAQ. Once you learn how this weapon works, dodging its projectile and "tracers" will be straightforward as using the arm will be.
The rocket launcher is a powerful projectile weapon that can easily kill in one hit. However, unlike rocket launchers in modern games, the rockets this weapon fires are slow; so slow that a straferunning player can easily outrun a rocket that he had just fired, given enough space. Rockets, therefore, can be dodged with ease at long range. Another drawback to the rocket launcher is that it can produce a great amount of splash damage to the player firing it, rendering it unsuitable for close encounters.
This weapon is most useful at long range combat or chasing down a fast moving player, especially one armed with an SSG that, with its lengthy reload time, interrupts the opponent's responses. The chaingun fires small shots rapidly and thus has the ability to constantly disorient an opponent with the near-persistent amount of red-screen build-up it causes. The first two shots of the chaingun are perfectly dead-on, while subsequent shots are not, due to a phenomenon referred to as "chaingun recoil". Therefore, to maintain accuracy with the chaingun, one must tap the fire button repeatedly instead of depressing to fire the first accurate shots repeatedly.
Another projectile weapon, the plasma rifle shoots fast-moving blue balls of plasma that can cause a considerable amount of damage, and is therefore one of the more efficient weapons. The pinnacle of its efficiency is seen at close-range and medium-range combat, where it is difficult for the adversary to escape the flying projectiles. The main disadvantage of the plasma rifle is that the projectiles can obstruct the view of the user and can be dodged with reasonable ease from a distance.
The shotgun is a relatively weak weapon, especially when compared to the super shotgun, but its tight pellet spread does not subside at long range, making it a decent weapon to 'ping' people with at long ranges. It proves itself a fairly fast-killing weapon at close range, where its pellets are more concentrated. Rarely, a player, with a single shot, can kill an opponent who has 100% health and no armor.
There are three other weapons, all of which are of dubious value. The pistol, the default weapon any player first starts with in any given map, is far too slow to even consider using in a very practical sense. The fist and chainsaw are melee weapons that require their users to be as close as possible to another player to damage that player. However, the chainsaw can be a very deadly weapon, as can the fist be when a berserk pack is used; the power-up restores one's health to 100% and significantly boosts the damage dealt by the fist to the point where one blow is enough to kill and 'gib' an opponent.