Speed

In World of Warcraft, speed usually refers to a couple of things:
 * Weapon speed. This weapon property helps determine a weapon's damage per second. Speed in this case would be better rendered delay. A speed of 3.00 means there are 3 seconds between strikes.
 * Movement speed. The rate at which a character or mob moves.

Weapon speed
See attack speed and weapon speed.

Movement speed
Blizzard has been very careful to make movement speed an important element in World of Warcraft. Without any speed-modifying spells or abilities, no race has any advantage over any other in terms of speed. The fact that tauren and gnomes run at the same speed is perhaps explained by the idea that tauren are actually rather slow-moving, but their large size makes up for it, whereas gnomes are short-legged but hyperactive little runners. At any rate, all other things being equal, nobody can automatically beat anyone else in a foot race.

There are, however, many items, spells and abilities that modify one's movement speed, and it is perhaps a testimony to Blizzard's attention to detail in regards to speed that their effect is so noticeable.

Temporary effects that increase run speed include: Class abilities such as a rogue's Sprint and a druid's Dash (druid ability); item use effects such as Gnomish Rocket Boots or Nifty Stopwatch; and consumables like Swiftness Potion.

Sustained effects that increase movement speed include: Class abilities such as a hunter's Aspect of the Cheetah and a shaman's Ghost Wolf form; passive item effects like Runeblade of Baron Rivendare; and the Minor Speed boot enchant. If you die and become a ghost (or a wisp), you will also move more quickly than you did when alive until you resurrect (although this speed does not stack with any other effects).

Many abilities exist to decrease the movement speed of enemy units temporarily, as this is usually a tactically useful thing to do. These abilities are usually called snares.

A standard ground mount gives you a 60% speed increase, while an epic ground mount gives you a 100% speed increase. However, effects that give you movement bonuses while on foot won't help you when riding (although enemy effects that slow you down on foot can also do so while you're riding). Fortunately, there are also effects to help you ride faster, such as the ever-popular Carrot on a Stick trinket, Mithril Spurs that can be installed onto your boots, and the Riding Skill glove enchant. At levels 69 and 70, the Riding Crop is the best single speed booster, granting +10% speed (does NOT stack with other items/enchants), however, it does not work above level 70. These increase your movement speed when you're riding, but only when you're riding.

Because of Wisp Spirit, night elves become wisps when they die, meaning that they can return to their bodies more quickly than any other race and thus resurrect sooner. This effect is more pronounced the farther the Night Elf is from the graveyard when he/she dies, however, because the Wisp gets farther and farther ahead of the regular ghosts the longer the run is. The only other effect in the game that can make your ghost move faster is the level 15 Guild Perk The Quick and the Dead.

The mathematics behind how movement speed effects stack with each other is debated, but appears to be actually quite simple. Most on-foot speed effects do stack with each other, and likewise mounted speed effects stack with each other as well. The highest rank of Sprint, for example, temporarily increases a Rogue's movement speed by 70% (that is, the Rogue is moving 170% of the speed of an unmodified character). If this Rogue got the Minor Speed enchantment on his boots, which gives an 8% speed bonus (allowing the Rogue to run 108% of the speed of an unmodified character when not Sprinting), he could then Sprint at 183.6% of the speed of an unmodified character (because 1.70 * 1.08 = 1.836). If a shaman were then to cast Frost Shock on the rogue (which temporarily slows movement by 50%, among other things), she would then be able to either catch or run away from the rogue, who would now be moving at 91.8% the speed of an unmodified character (because 1.836 * 0.50 = 0.918). Most experiments seem to bear out the fact that movement speed effects stack by multiplying in this way.

The paladin spell Pursuit of Justice (paladin talent), however, does not stack with other movement speed increases; this is explicitly stated in its tooltip.

The fastest sustainable land speed possible in the game pre-BC seems to consist of an epic mount (100% speed increase) in conjunction with the Riding Crop (10% mounted speed increase) for a total of 220.0% of the speed of an unmodified character (since 2.00 * 1.1 = 2.20). This same speed is achievable with the level 3 Guild Perk Mount Up. Shaman and druid travel forms also exhibit this multiplicative stacking; the 15% increase they receive from the PvP-Rare set yielding a total SUSTAINED speed of 161%. The fastest sustainable speed on foot, not in a travel form, might consist of a hunter with a Runeblade and the Minor Speed enchant under the influence of Aspect of the Cheetah, which would lead to a total of 151.6%. As for the fastest temporary speed, it is even less certain—it's possible to apply multiple speed-increasing effects, of course, but each one is subject to the global cooldown, so it would only be possible to activate a certain number of them before the first ones wore off. Imagine a Rogue with the Runeblade and Minor Speed, wearing Gnomish/Goblin Rocket Boots, in a party with a hunter using Aspect of the Pack, activating the Nifty Stopwatch, then Sprinting.

However, some speed enhancing effects will not stack together, even when it is not explicitly stated that they don't. For example,  and the Minor Speed Enchant are known not to stack, and the Riding Crop also does not stack with any other mounted speed enhancers. Sprint also overwrites the buff from Swiftness Potions, preventing both from being used together.

Known speeds
When walking is enabled (vs. run), then all ground speeds are 2.5 yards per second (35%), both mounted and unmounted. When moving backwards, the speed is always 4.5 yd./s. (64%), unless on the ground and walking (2.5 yd./s.)

Macro to show current speed
You can use the following macro while in game to show your current speed. /script ChatFrame1:AddMessage(string.format("Player speed: %d%%", (GetUnitSpeed("Player") / 7) * 100))

Another macro to show current speed
Here's another macro written a bit differently and with some added speed conversions. /script x=GetUnitSpeed("player") DEFAULT_CHAT_FRAME:AddMessage(format("Speed: %d yds/sec, %.1f kph, %.1f mph, %d%% normal run speed",x,x*3600/1093.6133,x*45/22,x/7*100))

Trivia
Characters are extremely fast by real-world standards. The standard running speed is 7 yards per second, or a mile every 4 minutes and 12 seconds (2:37 per kilometer). That's the speed of an excellent collegiate runner, carrying no gear, on a track - and a character can keep this up indefinitely in very harsh terrain in full plate armor! A rogue using Sprint will run 100 meters in 9.2 seconds, while the current men's world record for that distance is 9.58 seconds.