Overview
Train has a few meanings in World of Warcraft and MMORPGs:
- To improve some ability, power, or skill by paying a Trainer or reading a training Book.
- To use aggro to draw a bunch of mobs so they attack another group of Player characters.
- To have several players use the auto-follow feature to travel in a long line.
- The silly emote: /train
How Do I Train A Skill?
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Training Mobs
In an MMORPG, it is often possible to grief another player or party by aggroing a number of nearby mobs that would ordinarily not be close enough to see the players, then running toward the players so that the mobs chase you into them and attack them. If done at an inopportune moment, such as when the other player or party is currently fighting a close fight, this can turn their victory into a defeat and possibly a party wipe, making "training" a way for a single player to use the environment to defeat an entire party of the opposite faction -- or even of the same faction.
The developers of World of Warcraft seem to have realized that this is undesirable -- even on a PvP server, the fact that one player could successfully and repeatedly grief an entire party of other players could have a strong negative impact on a lot of people's gaming experiences. There are, therefore, countermeasures against mob training in WoW:
- Leashing: This is what it's called when a mob suddenly clears its aggro list, switches to Evade mode and runs quickly back to its spawn point, where it resets. During Evade mode, the mob is immune to all damage, cannot acquire aggro, and recovers health/mana/etc. at tremendous rates.
- Outside of instances, mobs have a critical distance from their spawn points that they won't exceed. A mob that is chasing a player and reaches this distance will break off pursuit and leash back to its spawn point. Inside instances, mobs usually don't have this limit.
- In an instance, the only players present are all in the same party or raid group, so there is no way for a player not in the party/group to train mobs on them.
- A mob that is targeting a player will keep targeting that one until another player moves to the to of its aggro list for some reason -- the targeted player can die,
[Vanish], become invisible, use 
[Feign Death], etc., or another player can do enough damage to the mob to supplant the first player at the top of the mob's list. If there is no one else on its list, the mob leashes back to its spawn point.
It is therefore difficult to train mobs on anybody in WoW; they will only attack the target party if someone in the target party (although this does include Hunter and Warlock pets) hits the mob as it goes by, and then only if the would-be mob trainer dies before the mob has reached its critical distance from its spawn point. If the mob trainer escapes, the mob(s) will just leash back to their spawn points, ignoring the target party at least until that time. They will continue to ignore the target party at least until they enter the mobs' aggro radius.
Train Emote
- When a player types in /train, they will start making train noises and movements. "Woo wooooo! Chugga chugga chugga..."