In Arena of Valor the key to being successful is to have a well-rounded team. There are six different categories of heroes available and each is able to fill multiple roles depending on how you want to utilise them. Below is a list of the six categories.
Tank
These characters typically are strong and solid with plenty of hit points and are able to stand in the front lines and soak up lots of damage. As a tank character you will be able to 'peel' enemies off weaker members of your team by using abilities to interrupt the attack or redirect it to you. Tank characters buy defensive items that will add to their hit points, armour, and magic resist.
Warrior
Although not as solid as tanks warriors are able to serve a similar role but what they lack in the survivability of tanks they make up for in being able to deal more damage. Warriors are particularly effective in close range melee battles and playing them requires knowing when to disengage from fights so you don't die. As s warrior character you will be looking to balance your purchases between survivablility and damage dealing.
Assassin
Assassins are stealthy characters who are effective at 'ganking' the enemy using underhand means. They are the opposite of tanks in that they lack survivability so if you play as an assassin and you don't kill your target the chances are you will be the one getting killed. Assassins have abilities that prioritise both damage dealing and mobility.
Mage
Mages are characters that cover a broad category but they all have one thing in common and that is that they focus on building their ability power so they are able to increase the damage done by their spells. Mages and their magical attacks are particularly effective when dealing with the enemy team when they are building lots of items that increase armour. However their low base hitpoints makes them easy targets for assasssins.
Marksman
Marksman are pure ranged damage dealers who stay in the back-lines attacking whoever comes within their range. As a Marksman most of your damage will come from ranged auto-attacks and you will be looking to build attack damage and improve your critical strike chance. Similar to mages, marksmen are easy targets if they encounter any other damage-dealing classes they are unable to escape.
Support
Like mages, support covers a broad category but they are basically characters who primarily assist the team in any way they can. If you play as a support character you will have disruptive abilities that can be used to snare, grab, stun, or 'crowd control' the enemy team. Support characters also tend to have ablilities that will enable you to heal or sheild your teammates.