Feral: summons three treants at targeted location, cast [Bash]
Guardian: summons three treants at targeted location, cast [Taunt]
Restoration: summons only one treant at caster location, casts [Healing Touch]
Force of Nature causes a pet bar to appear when cast, allowing the druid to choose which enemy they would like the treants to engage.
If cast when the druid is not on an enemy's aggro table, or is somehow engaged with a mob, it is common for the treants to return to the caster without attacking anything, unless the mob was aggroed by having the treants cast directly onto it.
If the treants are the only ones attacking a target for the entire duration of the fight and end up killing it, the target will not yield any Experience or Honor. This is especially important to keep in mind when helping lower-level players complete an instance - if your treants kill a boss by themselves, no-one will get quest credit for it.
Treants' health and damage scales with the druid they belong to.
Tips and tactics
Casting Faerie Fire on a mob that your treants are attacking will increase their damage output.
Treants can provide a very substantial amount of DPS in a boss fight, but it is important to time the cast so that the treants are out of danger of being annihilated by AoE. It is also important that you cast them at the back of the boss, so that they don't cause the boss to parry, and so they don't die due to cleaves.
Using treants effectively in PvP can be difficult, and should be approached differently for each opposing class. Generally speaking, it is a good idea to use them in a panic situation when your opponent is low on mana, running away from something or someone, or focused on healing/damaging someone else.
Against priests of all specs, it is a good idea to cast treants soon after they cast Psychic Scream so that they cannot instantly nullify your treants.
Patch 3.3.0 (08-Dec-2009): Health on the treants has been increased.
/Patch 3.0.2 (14-Oct-2008): Cooldown reduced to 2 minutes, down from 3.
Patch 2.3.0 (13-Nov-2007): This ability will no longer automatically break stealth on victims in its casting area. Nearby stealthed characters may still be attacked if they are too close to the summoned Treants.
Patch 2.2.0 (25-Sep-2007): Using this spell in a neutral town will no longer cause its victim to be attacked by town guards. It also will no longer cause nearby party members to lose stealth.
Patch 2.1.2 (2007-06-19): This spell will no longer causes players it is cast on to enter combat with nearby creatures. All of the treants summoned will now attack a neutral creature they are summoned on.
Patch 2.1.0 (22-May-2007): The Treants summoned by this spell will now correctly attack your target even if it is neutral or non-aggressive. These pets will now come into the game with full health, including that gained from a percentage of master’s stamina.