Itemization in Dota is probably the hardest aspect of the game to learn, since the mechanics of how items interact with each other are based on quirks and distortions of the WC3 engine. There problem isn’t a single person who knows every mechanic. Not even Icefrog. I’ll go through some of the major item mechanics though and how to itemize the various player positions through out the game.
I think the three critical mechanics are illusion-mechanics, magic immunity mechanics, and item-stacking mechanics. Illusions come from three sources: the item Manta Style, illusion runes, and a handful of hero spells. The important thing to know about illusions is that they benefit from only: the base stats of strength, ability, and intelligence; raw HP, critical strike multipliers and movement speed. Nothing else benefits illusions (until advanced gameplay, at least.)
Magic immunity is an important to know for both your defensive and offensive gameplay. Magic Immunity is offered by a few sources, but Black King Bar (BKB) is the most prevalent. Magic immunity blocks all magic-based damage and the effects of non-ultimate abilities, such as stuns. This means that the effects of ultimate abilities will penetrate magic immunity, but a twist is that magic immunity will clear effects already on your hero. So, if you currently magic immune, you may be stunned by a Beastmaster’s Primal Roar. You will not take the 200 - 300 damage that you’d normally take from the ability. Slardar’s Amplify Damage will reduce your armour through magic immunity, but if you activate it after being affected by Amplify Damage, you’ll remove the Amplify Damage from yourself. Some non-ultimate abilities will penetrate magic immunity, but that isn’t important at this level.
How items stack is pretty simple once you know the rules. The rules were simplified in the transition from Dota 1 to Dota 2, so it’s now much easier. The first rule is how Unique Attack Modifiers interact with each other. It’s only possible to use a single Unique Attack Modifier at a time and items or abilities that are Unique Attack Modifiers are clearly labeled as such. Your heroes active Unique Attack Modifier will be the highest of this priority: Proc-based Hero Skills, Proc-based Items, always-on Hero Skills, always-on Items.
Percentage-chance items and abilities such as critical strike chances and block chances work as follows: for each applicable instance (such as delivering an attack and critical strike) there is a roll on each of your sources. Whatever roll succeeds is that effect that is used. In the case of more than one roll succeeding, the strongest version of that effect is used. A 400% critical strike is override a 200% critical strike. A 20-damage block will override a 10-damage block. An exception to this is evasion items. For these items, only the highest source of evasion is rolled against. Not all sources. Items that increase statistics (strength, armour, whatever) will add together straight up. Though an exception is that you may only benefit from the movement speed of a single pair of boots.
Itemizing for the various rolls requires understanding the costs of items. Items have two costs: their gold cost and the opportunity cost of being unable to put a different item in that inventory slot (without taking a gold hit for selling the item.) In general, Support roles want to focus on items with efficient gold costs but high opportunity costs. Carry roles want to focus on gold inefficient items but with low opportunity costs. Gankers want to strike a balance. All roles get regeneration and stat items at the very beginning of the game and then a pair of boots as their first item.
Itemizing support heroes is the easiest of them all. The number of gold efficient items is fairly limited and players in this role don’t have enough gold to buy higher level items. Support players should buy wards, which allow them to locate heroes on the map, utility items such as dust of appearance to detect invisible heroes or a courier to transport items at the beginning of the game. All heroes need a way to regenerate mana and HP so they’re able to stay out in the field being useful. An effective item for this on a support hero is an Urn of Shadows or Magic Wand. A support player will generally begin the game by purchasing a courier, buy wards throughout the game, and buy a ghost sceptre as their final item. The support player with the greatest gold income (generally a jungle hero or offlane hero) will purchase a Mekansm to assist defensive counterganks and during team fights.
Itemizing a carry hero is also very easy. After getting a pair of boots, you build a farming item. What determines a farming item depends on the nature of your hero, but the most common are Battlefury, Linkens Sphere and Maelstorm. All of these items increase the rate you earn gold, which compounds through the game. Melee heroes use Battlefury. Heroes that use a lot of spells to farm get Linkens. Maelstorm provides heroes a means of killing large numbers of creeps quickly without relying on AoE spells or the melee-only Battlefury. A carry player follows up the farming item with a Black King Bar. The next item is a damage item such as Monkey King Bar or Daedalus. The final item is a survivability item such as Heart of Tarrasque, Assault Cuirass, or Butterfly. If the game goes long enough, a second damage or survivability item is picked up, depending on your need.
Itemizing a ganker is fairly open. You’re trying to fulfill two goals: being able to stay out on the map ganking and being stronger than any other hero on the map. Most ganker players will use a bottle for regeneration, since it synergies well with their rune control. An Urn of Shadows, Vladimir’s Offering or Vanguard isn’t uncommon though. With rune control, a Bottle and Magic Wand is sufficient for most heroes. The second goal is usually accomplished by getting an item balanced between being gold efficient and inventory efficient. It’s difficult to explain what to get since it depends on how your hero ganks. Strength heroes that rely on hero attacks will want an Armlet of Mordiggian or Medallion of Courage. Intelligence spell-based gankers may want an Orchid Malevolence or Dagon. A blink dagger is very common, since the mobility it provides prevents enemies from escaping. Drum on Endurance is another common item, since it provides a lot of stats to increase you hero’s power relative to everyone else and increases your mobility.
Regardless of your role, all heroes want to have a Town Portal Scroll. This item allows heroes to teleport to towers. You can use this to teleport to a lane to assist your gank or defense your carry. You can even use them to escape some ganks or move to another part of the map that has more creeps to farm. Extremely useful. As soon as you can do something useful in another lane, it’s important to get one and always have one.