Abilities are the unique skills used by the Heroes in Paragon. As each individual hero progresses through the match, they will gain XP (CXP) (experience) from killing enemy minions, structures, harvesters, and heroes. As you level up with your hero, they gain health & mana and unlock / level up their abilities.
Every hero has a basic attack, a passive ability, and four active abilities. Passive abilities provided a constant benefit and won't be covered further in this guide. For simplicity, whenever the word "ability" is used from this point on, assume we are talking about the four actives.
You begin the game with both your basic attack 
and secondary 
ability unlocked, and are also able to choose a third ability that you would like to start the game with Q or E. You unlock and level up abilities by holding Ctrl + the use key for that ability Q, E, R, , or .
As you level level up your Hero, you can unlock the rest of your Hero's abilities, including their Ultimate (R), which becomes available at level 5. The Ultimate is the most powerful ability in a hero's kit, and often defines the type of hero you are playing. As your hero continues to level, they can further upgrade their basic attack and four abilities. Additional ability levels typically provide extra damage and allow players to use the ability more often.
Note: There are some further restrictions as to when a given ability can be leveled up, and how many times. The in-game HUD will display how many levels each ability has, as well as which can be leveled up when a player reaches that point.
Using Abilities[]
All heroes have a basic attack that is accessed by pressing . This can be either a ranged or melee attack, the effectiveness of which will vary widely depending on the class of the hero. Many abilities in the game require players to first target and then confirm the ability to use it. Targeted abilities require you to press the key for the ability, aim it, and then press to confirm it. There are also abilities that do not require targeting. These can either activate passively (automatically) or by simply pressing the key that is mapped to that ability.
Whenever you use a non-basic attack ability, it will use some of that your hero's mana. Every player has a finite pool of mana that gradually replenishes itself, represented by the bluish-purple bar underneath the health bar. The purpose of mana is to regulate how often powerful hero abilities can be used during combat.
After an ability is used, it goes on cooldown for a period of time. The power of an ability often determines the cooldown length. When the cooldown timer has reset, the ability will be ready to fire again...assuming the player has enough mana to use it.