Troops refers to your army and what you choose to train. It is often very tempting to train a little bit of every troops type but there are definitely troops that are more useful than others.
Please see Troop Comparison for a quick look at the different troops and information regarding training times.
Troop Targeting Priority Chart.
Your starting troop. Very weak but very quick and easy to build. Many players often build exorbitant numbers to try and inflate their power. These are often used as meat shields - they die so that your other, more valuable troops survive. Concripts are also quite often used to attack level 10 Grodz camp. You tend to lose the majority but they are easily replaced.
Your starting pack mule. These are slow and carry very little. Don't bother building too many of these - maybe enough to complete the quest.
These are used to spy on an enemy player. If successful they will provide certain information including amount of resources, whether they are defending etc. You get more information the higher the level of clairvoyance you have. Some players will build enormous numbers of this troop and use them for attacking. Their high speed makes them quite effective against some troops but they do tend to die pretty quickly and easily.
A melee troop with higher stats compared to the Conscripts. I wouldn't bother building these or if you don't want to build Conscripts then you can just build Halberdiers.
Another melee troop. Improved stats compared to Halberdiers and Conscripts but also with a higher training price. You are better off using the resources to build other troops.
This is one troops which you will want to have. So long as they have adequate meatshield protecting them from any fast melee troops your Longbowmen will be able to stand back and shoot down the enemy. Cons/Hals/Mins/Giants are all adequate meatshields. LBM are also effective in taking down Swift Strike Dragons and Battle Dragons.
Swift Strike Dragons (SSDs)
Your starting dragon. They are, as their name suggest, very quick and are ideal for farming other players in large numbers. They are pretty squishy though and if you send them with slower troops they will engage the enemy first and usually be your first losses.
Battle Dragons (BDs)
Battle dragons or BD should definitely be one of the troops you focus on. They are pretty much effective against anything and are pretty fast to boot. They are very metal heavy so be prepared to farm as much as you can get in order to build as many of these as you can.
Armored Transports (ATs)
Faster, more durable and able to carry more than Porters. You definitely want a few of these. ATs will allow you to transport resources to other players, send them out to prevent an attacker from taking them or to help carry large amounts of resources if you send them with your army when attacking other players.
The ultimate meatshield. Very expensive and slow to train. They will slow any marches you send them with quite a lot but their high health and defense provides your LBM and other range troops the best defense. As they are so slow most people use them when defending although they can be used to attack as well.
Fire Mirrors (FMs, Mirrors)
Your ultimate ranged troop but even slower than Giants. Be prepared for long marches if you send them to attack and have no speedups. Make sure you have some meatshields in place so that they can do their work from a distance.
Fangtooth (FTs, Fangs)
Only available if you have the Water Dragon and have built Pearl Farms for a new resource - Pearls. FTs are both melee and ranged troops and are very effective. An army of FTs and Giants are very effective. They are pretty slow to train and the number you can train is limited by how many pearls your farms can produce. You also cannot steal pearls from other players, currently.
Marsh Horror (MHs, Horrors)
Only available if you have the Nightshade Dragon and have built Nurseries for a new resource - Seeds. MHs are much like Giants except more powerful and more expensive to train. They are slow to train and the number you can train is limited by how many seeds your nurseries can produce. You also cannot steal seeds from other players, currently.
Granite Ogres (GOs, Ogres)
Only available if you have the appropriate Dragon and have built provision for a new resource - Geodes. Granite Ogres are beings of living rock, whose stony bodies can withstand all but the heaviest attacks.
Chariots of Fire (CoFs, Chariots)
Only available if you have the appropriate Dragon and have built provision for a new resource - Brimstone. Blazing into battle with incredible ranged attack. Chariots of Fire are lethal fighters that withstand the most heated of moments.
Venge Wyrm (VWs, Wyrms)
Venge Wyrms are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. They are quite fast and powerful but they have no load so they can't carry any resources.
Lightning Cannon (LCs, Cannons)
Lightning Cannons are similiar to Fangtooths in that they are troops with both melee and ranged capabilities. However, they are more designed for defense due to their "special abilities." +25% damage to Fangtooths and +100% damage when defending.
Storm Drake (SDs, Storms)
Storm Drakes are slightly slower then Battle Dragons but twice as powerful. Designed for killing ranged troops, SDs have a "special ability." +50% damage to all ranged units.
Lava Jaws (LJs)
Lava Jaws are from the volcanic depths of Atlantis, these reptilian beasts can project molten fire which will melt anything in their way.
Petrified Titan (PTs)
Petrified Titans are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. When these mighty protectors of nature are provoked they are unstoppable. +75% Damage to Lava Jaws, Longbowmen & Fire Mirrors.
Dimensional Ruiner (DRs)
Dimensional Ruiners are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. These inhuman defilers are brought forth purely to destroy. When attacked, -75% Fire Mirror & Lava Jaw Damage, & -50% Lightening Canon Damage.
Dragon Cavalier (DCs, Cavs)
Dragon Cavalier are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. With incredible speed and damage, the Dragon Cavalier is the overlord of the sky. +75% Damage to all Ranged units.
Arctic Leviathan (ALs)
Arctic Leviathan are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. These massive creatures dwell in the darkest depths of the North. They are most known for their massive size and greater range.
Venom Dragon (VDs)
Venom Dragon are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. With a toxic aura able to disintegrate hefty armor, Venom Dragon unleash +200% damage when attacking Granite Orges.
Shadow Stalker (SSk, Stalkers)
Shadow Stalker are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. The Shadow Stalker is a master of speed and stealth. Their first attack in combat deals DOUBLE DAMAGE.
Sand Striders (SSd, Striders)
Sand Striders are exclusive troops from special events or sometimes can be bought, with rubies, through special shop items. The ultimate fast attack forces with capabilities to counter long and short range foes. They can recover in Healing Pools with -50% recovery time.
Hoplite are a powerful force when in a large group. Every 2000 Hoplites can reduce 1% damage, up to a max of 80%.
War Scarab (WS, Scarab)
War Scarabs are exclusive troops from special events. War Scarabs have the special ability of taunting enemies and if hit with melee or ranged attacks their prickly spikes return up to 50% of their health on damage taken.
Thunder Golem (TG)
Thunder Golems are exclusive troops from special events. Thunder Golems have the special ability of all stats x2 when defending. Also, Lighting Cannon, Lava Jaw, & Arctic Leviathan attacks -50%
Toxic Vipers (TV, Vipers)