Inventory
You have 99 bottles of beerA inventory system allows the player to store items and carry them around.
Unlimited inventories
This inventory puts no limit on how many things a player can carry around.
Limited Inventories
This limit what items they player can carry.
Rule of 99
The player can store a fixed number of items of each but other than that there is no limit. The 99 is from the fact that early role playing games limited it to max two digits.
The Weighted Inventory
Each item has a weight.
The Visual Grid
This type of inventory show the inventory as a grid for the player. Each item takes up a number of slots. As an example a a knife takes 1 slot, a sword 1x3 and a shield 2x2.
Slots
There is a number of slots in the inventory and each slot allows certain types of items to be in it. Some items might stack in the slot.
Reference
Designing an RPG Inventory System That Fits: Preliminary Steps