From https://theethereum.wiki/w/index.php/ERC20_Token_Standard

Ethereum-ERC20

 1 contract TokenContractFragment {
 2  
 3     // Balances for each account
 4     mapping(address => uint256) balances;
 5  
 6     // Owner of account approves the transfer of an amount to another account
 7     mapping(address => mapping (address => uint256)) allowed;
 8  
 9     // What is the balance of a particular account?
10     function balanceOf(address _owner) constant returns (uint256 balance) {
11         return balances[_owner];
12     }
13  
14     // Transfer the balance from owner's account to another account
15     function transfer(address _to, uint256 _amount) returns (bool success) {
16         if (balances[msg.sender] >= _amount 
17             && _amount > 0
18             && balances[_to] + _amount > balances[_to]) {
19             balances[msg.sender] -= _amount;
20             balances[_to] += _amount;
21             return true;
22         } else {
23             return false;
24         }
25     }
26  
27     // Send _value amount of tokens from address _from to address _to
28     // The transferFrom method is used for a withdraw workflow, allowing contracts to send
29     // tokens on your behalf, for example to "deposit" to a contract address and/or to charge
30     // fees in sub-currencies; the command should fail unless the _from account has
31     // deliberately authorized the sender of the message via some mechanism; we propose
32     // these standardized APIs for approval:
33     function transferFrom(
34         address _from,
35         address _to,
36         uint256 _amount
37     ) returns (bool success) {
38         if (balances[_from] >= _amount
39             && allowed[_from][msg.sender] >= _amount
40             && _amount > 0
41             && balances[_to] + _amount > balances[_to]) {
42             balances[_from] -= _amount;
43             allowed[_from][msg.sender] -= _amount;
44             balances[_to] += _amount;
45             return true;
46         } else {
47             return false;
48         }
49     }
50  
51     // Allow _spender to withdraw from your account, multiple times, up to the _value amount.
52     // If this function is called again it overwrites the current allowance with _value.
53     function approve(address _spender, uint256 _amount) returns (bool success) {
54         allowed[msg.sender][_spender] = _amount;
55         return true;
56     }
57 }