ERC-721 tokens are a type of non-fungible token (NFT) standard on the Ethereum blockchain. Unlike ERC-20 tokens, which are fungible and interchangeable, ERC-721 tokens represent unique assets that cannot be exchanged on a one-to-one basis. This uniqueness makes ERC-721 tokens ideal for representing ownership of digital art, collectibles, virtual real estate, and other one-of-a-kind items.

1. **Key Features of ERC-721 Tokens**

• Non-Fungibility: Each ERC-721 token has distinct properties and value, making it non-fungible. This means that no two tokens are identical, and they cannot be exchanged for one another like traditional currencies or fungible tokens.
• Ownership: ERC-721 tokens provide a clear and verifiable proof of ownership on the blockchain, allowing users to buy, sell, and trade unique digital assets.
• Metadata: Each ERC-721 token can have associated metadata, which can include information such as the name, description, and image of the asset it represents.

2. **Common Functions of ERC-721 Tokens**

• balanceOf: Returns the number of NFTs owned by a specific address.
• ownerOf: Returns the owner of a specific token ID.
• transferFrom: Transfers ownership of a specific token from one address to another.
• approve: Allows a specified address to transfer a token on behalf of the owner.
• getApproved: Returns the approved address for a specific token ID.
• setApprovalForAll: Enables or disables approval for a given operator to manage all of the caller's tokens.
• isApprovedForAll: Checks if an operator is allowed to manage all tokens of the owner.

3. **Sample Code: An ERC-721 Token Implementation**

Below is a simple implementation of an ERC-721 token in Solidity:

solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/utils/Counters.sol";

contract MyNFT is ERC721 {
    using Counters for Counters.Counter;
    Counters.Counter private _tokenIdCounter;

    constructor() ERC721("MyNFT", "MNFT") {}

    function mint(address to) public {
        uint256 tokenId = _tokenIdCounter.current();
        _mint(to, tokenId);
        _tokenIdCounter.increment();
    }
}

This MyNFT contract implements the basic functionalities of an ERC-721 token:

• Constructor: Initializes the token with a name ("MyNFT") and a symbol ("MNFT").
• mint: Allows the creation of new NFTs. The function mints a new token with a unique ID and assigns it to the specified address.

4. **Use Cases for ERC-721 Tokens**

ERC-721 tokens have a wide range of applications, including:

• Digital Art: Artists can create and sell unique digital artworks as NFTs, allowing them to retain ownership and provenance.
• Collectibles: Virtual collectibles, such as trading cards or in-game items, can be represented as ERC-721 tokens, making them scarce and tradable.
• Gaming: In blockchain-based games, players can own unique in-game assets, characters, or skins as ERC-721 tokens.
• Real Estate: Virtual real estate can be tokenized as ERC-721 tokens, allowing ownership and transfer of unique parcels of land in virtual worlds.

5. **Conclusion**

ERC-721 tokens represent a significant advancement in the world of digital assets, providing a standardized way to create and manage non-fungible tokens on the Ethereum blockchain. Their unique properties and capabilities have opened up new possibilities for ownership, creativity, and value in the digital space.