Tanks: Warrior, Chaotic Knight, Druid Bear Form Tanks are the meat shields of a team, and games can hardly start without one (unless one is way over the level limit of an area).

-Among the tanks, Warrior   is the easiest to start with, because he has higher threat output than the other two. Definitely a beginner's pick, but by no means worse than the other two! He has a very useful AoE buff that increases max health, a very valuable aid at all stages of the game.

-Chaotic Knight  is more complex and is generally harder to gain aggro with, but with a competent team, a Chaotic Knight increases overall DPS with his Armor Break debuff and Haste buffs, as well as having a minor heal spell for aggro and emergency healing. Not recommended for beginners.

-Druid Bears  have the highest health and provide higher DPS than either of the other two, but they require decent gear to function well (most Druid tanks start off as other roles until they gain the gears required to tank, then respec).

Melee DPS: Phantom Stalker, Barbarian, Shadowblade, Druid Wolf Form Melee DPS are generally harder to play with than Ranged DPS because they have to be next to a target to deal damage, thus having less reaction time windows and having to be more aware of things such as cleaves.

-The Phantom Stalker  dishes out the highest single target DPS in game, at the cost of being one of the most squishy heroes. Not advisable for first timers.

-Barbarians  have a much-loved Warcry AoE buff, which increases the health regeneration of allies. They also have good single-target and AoE DPS capabilities. Generally not the main DPS of the team, but a welcome addition nonetheless. 

-The 'Warlock' class of TKoK, Shadowblades  deal with debuffs. They can leech attack speed, drain life and extend debuff durations on targets. Shadowblades have two signature abilities, Unholy Embrace, which deals damage at the end of the duration based on how much damage was dealt to the target while the debuff was on (essentially doubling damage dealt), and Time Strike, which enables the Shadowblade to attack all enemies in an AoE area at the cost of mana-per-attack.

-Druid Wolves  specialize in single-target DPS, though they can deal a little AoE damage at a pinch. Like the Druid Bears, they require decent gear to perform well. Their 'Howl of Terror' reduces nearby enemies' attack damage, easing the healer and tank's jobs.

Ranged DPS: Arcanist, Hydromancer, Pyromancer, Ranger, Druid Caster Form Easiest to play, but not advised to play with at the beginning (easy starts make learning harder classes all the more harder).

-Arcanists  specialize in AoE damage, with periodic damage bursts and a buff to increase self magic damage dealt. Efficient in dealing with large clusters of mobs/adds. Has a channeling spell (the only one in all the heroes), thus requiring good positioning to avoid damage.

-Hydromancers  are among the best supporting DPS heroes. They have a powerful Ice Barrier, which places a shield on a target (and a lesser shield on all surrounding allies if upgraded) that absorbs damage. Hydromancers have a spell that can heal or damage over time, depending on target. They also have a pseudo-crowd-control freeze. Good choice for beginners.

-Pyromancers  deal loads of damage (AoE or single-target based on build). They also have Burning Desire, an AoE buff that increases mana regeneration (extremely useful at higher levels). Due to massive damage output, well-geared Pyromancers may draw aggro faster than less-geared tanks can take.

-Rangers  are another good choice for beginners, with decent AoE DPS, a Frost Arrow that can either AoE heal or AoE freeze/slow and a Trap ability that has Fire (damage), Frost (slow), Net (Root) and Poison (DoT) choices. One of the best choices for beginners (in my opinion).

-Druid Casters  have excellent crowd control and AoE DPS, with a buff that increases Spell Power of a target ally. They have a very useful Entangling Roots ability that can be upgraded to ensnare targets in a large AoE, and a powerful Starfall ability that, unlike its Warcraft III counterpart, does not need to be channeled. Simple and efficient.

Healers: Cleric, Paladin, Druid Treant Form Healers are the unsung heroes, the backbone of the team. Without healing, no team can survive long (unless they are way overpowered for the current encounter).

-Clerics  are the easiest healers to learn, as their skills are straightforward heals, buffs and AoE heals. Best choice for beginners to learn healing. A capable Cleric can singlehandedly keep a 7-man team alive.

-Paladins  are harder to play with because they are melee. Their skills require a little more brainpower than Clerics (Q puts a debuff on a target that makes anyone who hits it have a chance to heal and regen mana, and R deals damage, healing and restoring mana in an AoE if Q is on the target) but are as efficient as a Cleric's healing capacity. That said, a Paladin CAN be respecced as a single-target DPS dealer, although this requires end-game gear. 

-Druid Treants  start off weak (meager HoT and delayed single target heal), but function well enough in small (3-4 man) teams, or as secondary healer. At level 30, they gain a periodic 8 second AoE heal which enables multiboxing AFK gold farming.

(Note on Druids: Druids are generally classified into five builds, Tanks (Bear), Pure healer (Treant), Pure SP (Caster, with secondary healing capabilities), Pure AP (Wolf) and Versatility (Caster/Wolf/Treant). Focus on getting gear for one of these, if you choose to make a druid.)