What is the Message object in solidity?

msg / The message object is a global variable that holds data which will be important for access to the blockchain.

Quick Summary

#msg.data (bytes)
#msg.sender (address)
#msg.sig (bytes4)
#msg.value (uint)

msg.sender is most commonly used. It is the address of the original sender .

msg.value this usually holds Ether ( Wei (ether / 1e18) ) that is sent along with the message.

msg.sig will return a hash of the current function signature. So essentially,

   function boom( address _addr) return (bytes4 _sig) {
     return msg.sig 
   }

If you call the function boom(addr) it will return a hash that would look like33DB172D which would represent the hash of boom(address)

msg.data will return a payload in bytes. It holds the calldata as it is referred to in EVM terminology or the parameters that are passed to the function. If you pass in complex data structures into a function, the resulting msg.data will be structured in a complex way. It organizes the incoming params in bytes and delimits them with a 32 byte integer.