In Solidity smart contracts, there are 3 types of variable:

• Fixed-size types
• Variable-size types
• User-defined types

There are many different kind of variables, but the one I used the most are:

• uint, to represent numbers, like amount for ether / ERC20 token transfers
• address to specify recipient of transfers
• arrays, to represent collection of data
• mappings to also represent collection of data but this time when we want easy retrieval with a key
• struct to represent some structured data, like a user

The full video is available here: