###  ERC721
 Similar to ERC20 , ERC721 also allows smart contracts to function as tradeable tokens.
 The main difference is that, ERC20 tokens are fungible tokens where ERC721 are non-fungible tokens.
ERC20 token standard consist of six core functions :
- function transfer(address _to, uint256 _amount) returns (bool success)
- function transferFrom(address _from, address _to, uint256 _amount)returns (bool success)
- function balanceOf(address _owner) constant returns (uint256 balance)
- function approve(address _spender, uint256 _amount) returns (bool success)
- function allowance(address _owner, address _spender) constant returns (uint256 remaining)
- function totalSupply() constant returns (uint)

ERC721 consist of :
- function name() constant returns (string name)
- function symbol() constant returns (string symbol)
- function totalSupply() constant returns (uint256 supply)
- function balanceOf(address _owner) constant returns (uint balance)
- function ownerOf(uint256 _tokenId) constant returns (address owner) - returns address of owner of the token.
- function approve(address _to, uint256 _tokenId) - approves another party to transfer token on behalf of the owner.
- function takeOwnership(uint256 _tokenId) - used when a user has been _approved to own a certain amount of tokens and wishes to withdraw tokens from another user’s balance
- function transfer(address _to, uint256 _tokenId) - lets the owner of a token send it to another user
- function tokenOfOwnerByIndex(address _owner, uint256 _index) constant returns (uint tokenId) - (Optional)
- function tokenMetadata(uint256 _tokenId) constant returns (string infoUrl) - (Optional)