Local, state and global variables

Based on their scope, Solidity variables can be classified into:

Local variables: These variables are declared inside a function and are only accessible inside that function. They are not stored on the blockchain.
State variables: These variables are declared outside all functions but inside a contract. They can be accessed by all functions inside the contract. They are stored in the contract's storage, i.e. on the blockchain.
Global variables: These variables exist in the global namespace and are part of the Solidity language. They are used to access blockchain or transaction data in real-time. The exact value of variables like block.number may depend on the node you are using to connect to the blockchain. A complete reference to all global variables provided by Solidity can be found on their documentation.

Take a look at this code example for a clearer picture:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

contract VariableScopesInSolidity {

    // State variables are declared outside all functions
    uint public stateVariable = 123;

    function declareLocalVariable() public view {

        // Local variables are declared within a function
        uint localVariable = 123;        
    }

    function callAllVariables() public {

        // Local variable cannot be incremented
        // Compiler will throw an error
        localVariable += 1;

        // State variable can be incremented
        stateVariable += 1;

        // Global variables contain different types of blockchain data
        // Support for individual global variables may vary across blockchains 
        address sender = msg.sender; 
        uint Value = msg.value;
    }

}

📝 Note: Global variables basically read data in real-time. Different blockchains may implement the EVM in a different way. Therefore, the exact value of global variables may vary. In some cases the same global variable may return a totally different across different blockchains. For example, you can check out Scroll Network's documentation on the differences in their Opcode support.

Solidity in Foundry

Local, state and global variables