Any variables declared at the contract level (so not declared in a function or constructor), will automatically become contract storage. Contract storage is maintained on chain, so they retain their values between calls. These are declared so:
The counter is maintained for each deployed hitcount contract. When the contract is deployed, the contract storage is set to 1. Contract storage variable do not need an initializer; when it is not present, it is initialized to 0, or false if it is a bool.
A variable can be declared immutable. This means that it may only be modified in a constructor, and not in any other function or modifier.
This is purely a compiler syntax feature, the generated code is exactly the same.
Any contract storage variable which is declared public, automatically gets an accessor function. This function has the same name as the variable name. So, in the example above, the value of counter can retrieved by calling a function called counter, which returns uint.
If the type is either an array or a mapping, the key or array indices become arguments to the accessor function.
The accessor function may override a method on a base contract by specifying override. The base function must be virtual and have the same signature as the accessor. The override keyword only affects the accessor function, so it can only be used in combination with public variables and cannot be used to override a variable in the base contract.
Any contract storage variable can have its underlying contract storage cleared with the delete operator. This can be done on any type; a simple integer, an array element, or the entire array itself. Contract storage has to be cleared slot (i.e. primitive) at a time, so if there are many primitives, this can be costly.