Mana

Mana is the energy that heroes in DotA possess. Most abilities that are cast will require a specified amount of mana, known as the 'cost'. For instance, if you use an ability that costs 100 mana, your mana will lower by that amount. Under your hero's portrait, you can check your mana by looking at the blue fraction. The first number is how much mana you currently possess, and the second is the maximum that your hero can have. If you do not meet the mana requirements of an ability, you cannot use it.

Gaining Mana

The attribute Intelligence raises your mana, as well as the rate that it is replenished. There are also items like Energy Booster that directly affect your mana amount. Furthermore, items like Sobi Mask cause your mana to regenerate more quickly. Items like Clarity Potion temporarily increase the rate at which mana is regenerated. There are even abilities like Brilliance Aura and Chakra Magic that deal with mana.

There are three main types of mana regeneration. Firstly, percentage mana regeneration such as Void Stone and Sobi Mask allows the Hero to regenerate mana by a percentage of the Hero's base mana regeneration rate. Secondly, constant mana regeneration such as Brilliance Aura allows the user to regenerate mana at a constant rate. Thirdly, burst mana regeneration, while being a broad category, consists of abilities such as Arcane Ring's active ability and Chakra Magic. These abilities provide a burst of mana every time they are used.

Note that the percentage mana regeneration is based only on the base mana regeneration rate, and not on the current mana regeneration rate.

In addition, Chemical Rage allows Alchemist to have a Base Mana Regeneration of 3/7.5/12 mana/second. This is further amplified by percentage mana regeneration items such as Void Stone.

Targeting Mana

Some heroes take advantage of the need for mana by draining an enemy unit's manapool. The most notable would be heroes like Anti-Mage, Nerubian Assassin, Oblivion, and Keeper of the Light. Their respective skills (Mana Break and Mana Void, Mana Burn, Nether Ward and Mana Leak) are effective at taking away enemies' mana and either preventing or discouraging the casting of spells.

Mana Dependence

Mana is more of an issue on some heroes than on others. For instance, heroes like Skeleton King have disadvantages early because the Mana Cost of Storm Bolt is large compared to his entire Mana Pool. However, other heroes like Death Prophet have large mana pools and can easily spam their spells.

Addition

1 int gives 13 mana
All heroes have base mana based on this formula: Base int * 13