Items are objects and artifacts that provide special benefits and/or abilities to a god beyond their basic capabilities. They are only sold at the store at the fountains on each base. Items cost gold, and they can be sold back for a portion of the original cost (However, they can be sold for their full price as long as the player hasn't left their fountain or used the item.). Items are arranged in several categories: Consumable, Active and finally Passive items, which are subclassified in Starter, Offensive, Defensive and Utility items. Active and Passive items have branching tiers, that allow players to upgrade them into different, stronger versions of the original item which can then be upgraded once more. Players can carry more that one item from the same item tree but can't acquire two of the exact same item.
You can find a list with all current items sorted in different categories like type, tier or the stats they provide, here: List of items.
Consumables are one-time use items that are removed after they are activated. Each god can only carry two different consumables at a time, but they can carry multiple quantities of each one. You may also sell consumables back to the shop for half of the item's cost.
Actives grant the player a new ability like becoming invulnerable for a short duration or damaging nearby minions. As their name imply, Active items only take effect when activated, and once used they will go into a long cooldown before they can be used again. Each god can only have two unique active items at a time and they cannot be removed from your inventory after acquiring them and leaving your fountain or using them.
Passive items are the most prevalent. Each one provides a bonus to one or more stats like Health, Movement Speed, or Magical Protection. Each god can only hold up to six different passive items. Most Tier III Passive items also provide a unique Passive effect, like auras or On Basic hit effects. There are also a few Tier II items that provide unique effects. While most Passive items can be acquired by anyone, there are a number of items that are restricted to gods with a certain kind of Power (Physical or Magical), these being mostly the ones that provide Power, Lifesteal or Penetration bonuses.
Passive items are classified in different categories: Starter, Offensive, Defensive and Utility. Starter items are items with a single tier whose bonuses and unique passive effects are most useful on the early stages of a match, but gradually become less useful as time goes by. Offensive items provide boosts to stats related to attack and damage output (Physical & Magical Power, Attack Speed, Critical Strike Chance, etc.). Defensive items are the ones that provide bonus to stats related to defense and self-preservation (Physical & Magical Protection, Health and HP5.). Utility items provide you with boosts for misc. stats (Cooldown Reduction, MP5, Movement Speed, etc.) or that provide supportive and team beneficial bonuses (like Aura effects).