文章内容主要来自于 https://www.ethereum.org/token。笔者没有过英语4级，翻译的不好请见谅 ^_^

代币

我们将创建一个数字令牌（digital token）。以太坊生态系统中的token可以代表任何可替换可交易的物品：硬币，金币，游戏内物品等。任何token都以标准方式实现一些基本功能。这意味着您的token可以兼容以太坊钱包和其他任何使用形同标准的contract。
我们所说的代币，绝大多数都是token。

一个基于ERC20发行的token

先看一下代币的样子：
进入网站：https://etherscan.io/。随便找一个代币，点进去查看：
这里写图片描述

接下来看一下ERC20标准。以下是ERC20标准的接口声明：

// https://github.com/ethereum/EIPs/issues/20
contract ERC20 {

    string public constant name = "Token Name";//token的名字、代币名称
    string public constant symbol = "SYM"; // token的简写/代币符号
    uint8 public constant decimals = 18;  // 代币小数点位数，代币的最小单位， 18表示我们可以拥有 .0000000000000000001单位个代币。 官方推荐18。不要轻易改变

      function totalSupply() constant returns (uint totalSupply);//发行代币总量。 
      function balanceOf(address _owner) constant returns (uint balance);//查看对应账号的代币余额。 
      function transfer(address _to, uint _value) returns (bool success);//实现代币交易，用于给用户发送代币（从我们的账户里）。 
      function transferFrom(address _from, address _to, uint _value) returns (bool success);//实现代币用户之间的交易。 
      function approve(address _spender, uint _value) returns (bool success);//允许用户可花费的代币数。
      function allowance(address _owner, address _spender) constant returns (uint remaining);//控制代币的交易，如可交易账号及资产。 

      event Transfer(address indexed _from, address indexed _to, uint _value);
      event Approval(address indexed _owner, address indexed _spender, uint _value);
}

直接上代码：

pragma solidity ^0.4.16;

interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) external; }

contract LBL {
    // Public variables of the token
    string public name;
    string public symbol;
    uint8 public decimals = 18;
    // 18 decimals is the strongly suggested default, avoid changing it
    uint256 public totalSupply;

    // This creates an array with all balances
    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    // This generates a public event on the blockchain that will notify clients
    event Transfer(address indexed from, address indexed to, uint256 value);

    // This notifies clients about the amount burnt
    event Burn(address indexed from, uint256 value);

    /**
     * Constructor function
     *
     * Initializes contract with initial supply tokens to the creator of the contract
     */
    function LBL (
        uint256 initialSupply,
        string tokenName,
        string tokenSymbol
    ) public {
        totalSupply = initialSupply * 10 ** uint256(decimals);  // Update total supply with the decimal amount
        balanceOf[msg.sender] = totalSupply;                // Give the creator all initial tokens
        name = tokenName;                                   // Set the name for display purposes
        symbol = tokenSymbol;                               // Set the symbol for display purposes
    }

    /**
     * Internal transfer, only can be called by this contract
     */
    function _transfer(address _from, address _to, uint _value) internal {
        // Prevent transfer to 0x0 address. Use burn() instead
        require(_to != 0x0);
        // Check if the sender has enough
        require(balanceOf[_from] >= _value);
        // Check for overflows
        require(balanceOf[_to] + _value >= balanceOf[_to]);
        // Save this for an assertion in the future
        uint previousBalances = balanceOf[_from] + balanceOf[_to];
        // Subtract from the sender
        balanceOf[_from] -= _value;
        // Add the same to the recipient
        balanceOf[_to] += _value;
        emit Transfer(_from, _to, _value);
        // Asserts are used to use static analysis to find bugs in your code. They should never fail
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }

    /**
     * Transfer tokens
     *
     * Send `_value` tokens to `_to` from your account
     *
     * @param _to The address of the recipient
     * @param _value the amount to send
     */
    function transfer(address _to, uint256 _value) public {
        _transfer(msg.sender, _to, _value);
    }

    /**
     * Transfer tokens from other address
     *
     * Send `_value` tokens to `_to` on behalf of `_from`
     *
     * @param _from The address of the sender
     * @param _to The address of the recipient
     * @param _value the amount to send
     */
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);     // Check allowance
        allowance[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;
    }

    /**
     * Set allowance for other address
     *
     * Allows `_spender` to spend no more than `_value` tokens on your behalf
     *
     * @param _spender The address authorized to spend
     * @param _value the max amount they can spend
     */
    function approve(address _spender, uint256 _value) public
        returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        return true;
    }

    /**
     * Set allowance for other address and notify
     *
     * Allows `_spender` to spend no more than `_value` tokens on your behalf, and then ping the contract about it
     *
     * @param _spender The address authorized to spend
     * @param _value the max amount they can spend
     * @param _extraData some extra information to send to the approved contract
     */
    function approveAndCall(address _spender, uint256 _value, bytes _extraData)
        public
        returns (bool success) {
        tokenRecipient spender = tokenRecipient(_spender);
        if (approve(_spender, _value)) {
            spender.receiveApproval(msg.sender, _value, this, _extraData);
            return true;
        }
    }

    /**
     * Destroy tokens
     *
     * Remove `_value` tokens from the system irreversibly
     *
     * @param _value the amount of money to burn
     */
    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] >= _value);   // Check if the sender has enough
        balanceOf[msg.sender] -= _value;            // Subtract from the sender
        totalSupply -= _value;                      // Updates totalSupply
        emit Burn(msg.sender, _value);
        return true;
    }

    /**
     * Destroy tokens from other account
     *
     * Remove `_value` tokens from the system irreversibly on behalf of `_from`.
     *
     * @param _from the address of the sender
     * @param _value the amount of money to burn
     */
    function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(balanceOf[_from] >= _value);                // Check if the targeted balance is enough
        require(_value <= allowance[_from][msg.sender]);    // Check allowance
        balanceOf[_from] -= _value;                         // Subtract from the targeted balance
        allowance[_from][msg.sender] -= _value;             // Subtract from the sender's allowance
        totalSupply -= _value;                              // Update totalSupply
        emit Burn(_from, _value);
        return true;
    }
}