Ima just make a list of items, combined with their intended use/nieche, both for AP and AD, as that is what it boils down to in the end.
Offense: The item is primarily bought for damage purposes. (duh)
Defense: The item is primarily bought to help the user in bad situations or fights by their defensive capabilities.
Safety: The item is primarily bought to prevent the user from getting in bad situations. Usually some form of utility.
AD:
Usually, physical damage is built on these stats:
Flat AD, Attack Speed, Crit Chance, Crit Damage, Armor Penetration and Lethality
Attack Speed:
: Offense. Adds a nice amount of burst every few basic attacks. Basically the equivalent of
, if we're doing the compare game
: Offense. As an adc, the bolts give you more impact and overall damage in teamfights.
: Defense. The % damage reduction is pretty useful and pretty much the only reason it's bought. (Did you know it gives more AS than the other two?)
: Safety. More range on the first attack.
: Offense, Defense and Safety. It provides %hp damage on every basic attack and its unique active. It provides Lifesteal. And it's unique active helps you escape (or chase).
: Offense. You buy this item for the Sheen proc. Although the HP, Phage passive and overall stats are nice, this isn't the first choice when looking for these stats, should you not be a Sheen user.
: Offense.
Crit Chance: (which weren't already named)
: Offense. It's the only item to amplify crits.
: Offense, Safety. Up to 30% CDR, when combined with other Crit Chance items and mana sustain.
Armor Penetration and Lethality:
: Offense. Up to 20% increased damage.
: Defense. While defense is a bit of a stretch, it does provide you with considerable protection from a certain feat of your enemy; in this case: Health Sustain.
: Offense, Safety. Additional damage on your first attack and the ability to spot wards.
: Defense, Safety. Health is Defense, but considering the shield only works once per fight, I decided to put it into Safety too.
: Defense, Safety. The additional movement speed can be considered Defense aswell as Safety.
: Offense, Defense. Health and Movement Speed is useful for the player, while every AD champion in the team profits from the Armor Shred.
Flat AD: (which weren't named already)
: Double Defense. Sustain on every physical damage dealt, plus having 30% of taken damage be practically delayed is very strong.
: Offense, Defense. Health and the ability to cripple your opponent on every basic attack.
: Defense. Armor and a revive. Not sure what else to say about it.
: Offense, Defense. Honestly, more of an item for AP Divers/Assassins. However, it does provide Spell Vamp, similar to Deaths Dance and an active which deals additional damage and slows.
: Defense. A powerful tool against magic damage, it provides Magic Resist, an automatic Purple Shield, which is tied with increased AD, Lifesteal and Spell Vamp.
: Defense, Safety. Lifesteal, Magic Resist and the ability to break any CC (except Knock-Up to some extent and Stasis)
: Offense, Defense. AoE damage and lifesteal.
: Offense, Defense. AoE damage, health and a basic attack modifier.
: Defense. Health, Automatic Shield tied with size and Base AD increase.
: Defense. Lifesteal + Overheal.
: Offense, Safety. Additional damage on every basic attack + a bigger mana pool.
Now, AP items generally have less variety in offensive stats, but given the fact that magic damage is almost always tied to mana, it's just as important a stat to them as flat AP.
Flat AP, Magic Penetration, Mana
Magic Penetration:
: Safety. Other than the Magic Pen they are listed here for, Movement Speed is all they give.
: Offense, Defense. Health, plus a burn effect for % Max Health.
: Offense. All it gives is AP and Magic Pen.
: Defense. Though, actually not an AP item, it does increase magic damage dealt by 10%.
Mana:
: Offense, Defense. Increased AP for Mana, an activatable Shield and decreased spell costs.
: Offense, Defense, Safety. An AoE slow and Health.
: Offense. Increased damage tied to a basic attack.
: Offense, Safety. Another counter to Sustain and Mana restore on kills and assists.
: Defense. Health.
Flat AP:
: Defense, Safety. Magic Resist, and a passive Spellshield.
: Offense, Defense, Safety. Health, and the unique active can be used both offensively and defensively.
: Offense. Damage is all it gives.
: Offense. It provides AP the more kills you have.
: Offense. See Ludens.
: Offense, Defense. Health and a slow on all your abilities. Can also be used for Safety.
: Defense. Armor, and a 2.5 second Stasis.
: Offense. Gives Attack Speed and Magic Damage on-Hit on basic attacks.
There you have it. That's AD Itemization vs Mage itemization. If there's anything missing or wrong, tell me.