Poké Balls
Poké Balls are the most basic balls. They are useful for weaker Pokémon.  Additional Poké Balls can be acquired from the Shop or PokéStops.
Great Balls
Great Balls have a better catch rate than Poké Balls. They are most useful with Pokémon that aren't too strong.  Great Balls are unlocked at Level 12.  Additional Balls can be acquired from PokéStops.
Ultra Balls
Ultra Balls have a much great catch rate then Poké Balls or Great Balls.  These balls can be used on strongest of Pokémon.  Ultra Balls are unlocked at Level 20.  Additional Ultra Balls can be acquired from PokéStops.

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Premier Balls
Premier Balls can only be used after winning a Raid Battle against the Raid Boss.  After the encounter with the Raid Boss, any remaining balls are lost. 

Potion
Potions will restore 20 HP of a Pokémon. These are acquired from a Pokéstop. 

Super Potion
Super Potions will restore 50 HP of a Pokémon. These are acquired from a PokéStop.  They are unlocked at Level 10.
 
Hyper Potion
Hyper Potions will restore 200 HP of a Pokémon. These are acquired from a Pokéstop.  They are unlocked at Level 20.
  Max Potion
Max Potions will restore all the HP of a Pokémon. These are acquired from a Pokéstop.  They are unlocked at Level 20.  They can also be purchased from the Shop. 
 
Revive
Revives will revive a fainted Pokémon with half HP. These can be acquired from PokéStops.  Max Revives will restore a Pokémon with all of its HP.
 
Max Revive
Max Revives will revive a fainted Pokémon with all their HP. These can be acquired from PokéStops.  They can also be purchased from the Shop. 

Razz Berry
Razz Berries are used to make it easier to catch a wild Pokémon. They can be acquired from a PokéStop.
Nanab Berry
Nabab Berries are used to make a Pokémon more calmer. They can be acquired from a PokéStop.
Pinap Berry
Pinab Berries are used to make a Pokémon drop more candies after a successful catch. They can be acquired from a PokéStop.
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Golden Razz Berry
Golden Razz Berries are used to make it much easier to catch a wild Pokémon. They can be acquired from winning a Raid Battle.
Silver Pinap Berry
Silver Pinap Berries can be used to make it easier to catch a Pokémon and earn double candies.  These are acquired by completing research tasks.
Poffin
A poffin is used to make your Buddy excited.  An excited Buddy will give you more hearts per day and level up faster.

Sun Stone

Item used to evolve Gloom into Bellossom or Sunkern into Sunflora

King Rock
Item used to evolve Poliwhirl into Politoed or Slowpoke into Slowking

Metal Coat

Item used to evolve Onix into Steelix or Scyther into Scizor

 

Dragon Scale
Item used to evolve Seadra into Kingdra

 

 

Upgrade
Item used to evolve Porygon onto Porygon2 

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Sinnoh Stone
Item used to evolve certain Generation 1 to Generation 3 Pokémon into Sinnoh Region forms

  

 

Incubator- Infinity Uses
This egg Incubator will hatch an unlimited number of eggs.

 

Incubator- 3 Uses
These egg Incubators will hatch eggs. They are good for 3 uses before they break. Additional Incubators can be acquired from the Shop.

Super Incubator- 3 Uses
These egg Incubators will hatch eggs 1.3x faster than a regular Incubator. They are good for 3 uses before they break. Additional Incubators can be acquired from the Shop.

 

Eggs
Eggs are obtained from PokéStops. They can be 2km 5km, 7km, or 10km. The higher the km, the better the Pokémon inside the egg. Eggs need Incubators to be able to hatch. Trainers have to walk to hatch eggs.  Green Eggs are 2km, Orange Eggs are 5km, and Purple Eggs are 10km.  7km eggs can be obtained from friend's gifts.
    

 
Incense
Incense will attract wild Pokémon to the trainer. These are acquired from the Shop.

Lure Module
Lure Modules are attached to a PokéStop. They will attract wild Pokémon to the PokéStop. They last for 30 minutes. They can be purchased from the Shop. 
Magnetic Lure Module
Magnetic Lure Modules are attached to a PokéStop. They will attract Steel, Rock or Electric wild Pokémon to the PokéStop. They also allow Nosepass to evolve into Probopass and Magneton into Magnezone.  They last for 30 minutes. They can be purchased from the Shop.
Glacial Lure Module
Lure Modules are attached to a PokéStop. They will attract Ice and Water-type wild Pokémon to the PokéStop. They also allow Eevee to evolve into Glaceon.  They last for 30 minutes. They can be purchased from the Shop.
Mossy Lure Module
Lure Modules are attached to a PokéStop. They will attract Bug, Grass, and Poison Type wild Pokémon to the PokéStop. They also allow Eevee to evolve into Leafeon.  They last for 30 minutes. They can be purchased from the Shop.
Lucky Egg
Lucky Eggs will grant 30 minutes of double XP. They can be purchased from the Shop. 
 
Star Piece
Lucky Eggs will grant 30 minutes of 50% more Star Dust. They can be purchased from the Shop during special events.
 
Raid Pass
Trainers can acquire one free Raid Pass daily by spinning a PokéStop at a Gym.  These are used for in-person raids. 
Premium Battle Pass
Trainers can acquire Premium Raid Passes by purchasing them in the Shop.  These are used for in-person raids or used in Go Battle League for Premimum Rewards.
Remote Raid Pass
Remote Raid Passes are used for partcipating in a Raid outside of the range of a Gym.
Rare Candy
Rare Candy can be used to acquire a candy of any Pokémon.  They can be acquired by winning a Raid Battle or by completing Research. 
Fast TM
These can be used to teach a Pokémon a new fast move.  They can be acquired by winning a Raid Battle. 
Charged TM
These can be used to teach a Pokémon a new charged move.  They can be acquired by winning a Raid Battle or by completing Research. 

Gift
Gifts are found by spinning PokéStops and PokéStops from gyms.  Gifts can be sent to friends and trainers can open 20 per day. 

Team Medallion

The Team Medallion allows a trainer to change teams.  Can only be purchased only every 365 days.

 

 

Bag Upgrade
Upgrades to the bag cost 200 PokéCoins each. Each upgrade will give 50 additional spots for item storage. The max item count is 3000. 
Pokémon Storage Upgrade
Additional storage for Pokémon cost 200 PokéCoins each. Each upgrade increases the number of Pokémon you can carry by 50.  The max Pokémon storage is 3000.