While many popular top-level programming languages allow users to directly pass arguments to functions (function(argument1,argument2)), lower-level programming languages often use a stack to pass values to functions. The EVM uses a 256-bit register stack from which the most recent 16 items can be accessed or manipulated at once. In total, the stack can only hold 1024 items.
Because of these limitations, complicated opcodes instead use contract memoryto retrieve or pass data. However, memory is not persistent. When contract execution finishes, the memory contents will not be saved. Thus, while stack can be compared to function arguments, memory can be compared to declaring variables.
In order to store data indefinitely and make it accessible for future contract executions, one can use storage. Contract storage essentially acts as a public database, from which values can be read externally without having to send a transaction to the contract (no fees!). However, writing to storage is very expensive (as much as 6000x) compared to writing to memory.