Viper is fairly simple ranged hero and dominates mid. He has 2 passive abilities, one of which makes him great and last hitting and another which damages and slows attackers. One of his active abilities is known as an "orb" effect, meaning that it modifies your attack by making it deal more damage but costs mana, and you can ctrl+right click to make Viper always use it. His ultimate is a fairly straightforward nuke+slow. I would recommend building Shadowblade, Manta Style, and BKB on him. Shadowblade lets you escape+initiate safely, Manta Style increases your damage output by creating two illusions, and BKB (Black King Bar) helps you in teamfights against magic-heavy lineups.
Wraith King is a melee carry that has only one active ability, a fairly mana-heavy stun. He can be played as a tanky carry, and his Ultimate ability gives him an extra life, but you should try not to waste it, as it has an EXTREMELY long cooldown at early levels. His 2 other passive abilities grand him and nearby allies lifesteal, meaning that you and allies heal a small percentage of damage that you deal, and an ability that grants him a chance to critically strike. His only downside is that unlike most carries he's poor at Flash Farming, meaning quickly clearing out neutral creep camps unless you build a Maelstrom on him (you're familiar with the Jungle, right?).
Dazzle is a ranged support who has strong laning presence, and he can easily get first blood. His Q ability is a slow that deals damage and stuns at the end. It's great for aggressive laning strategies. His W ability puts a spell on a friendly unit for a short amount of time that makes them unable to die, leaving them at 1 hp. It can be very useful in certain scenarios, allowing an ally to escape or continue fighting for a short amount of time. His E ability heals him, his target, and friendly units around his target while damaging enemies. It's great for staying in lane and clearing out Creep waves when pushing. His ultimate is on a very short cooldown and grants allies armor and reduces enemy armor. It can be very powerful in the right scenarios.
Sand King is a melee support who is extremely powerful at many stages in the game. He has a stun that sends him rushing forward while stunning anyone in his path, an ability that creates a damaging sandstorm while turning him invisible, a passive that makes enemies you attack explode when they die, and an ultimate that creates a highly damaging series of shockwaves around Sand King after a short channeling duration, allowing him to walk around and use abilities while the waves damage enemies. The obvious downside here is that cancelling the channeling will completely waste your ultimate, whether you move or you get stunned by an enemy. That's why Sand King typically doesn't fight until he buys Blink Dagger, which allows him to channel his ultimate somewhere safe, then blink in once his ultimate activates.
