Tokens on Ethereum

Blockchain tokens existed before Ethereum.

• In some ways, the blockchain currency, Bitcoin, may be a token itself.
• There are many token platforms that were also developed on Bitcoin and other cryptocurrencies before Ethereum.
• The introduction of the first token standard on Ethereum led to an explosion of tokens.

Tokens standards (ERC20)

• The first standard was launched in November-2015 by Fabian Vogelsteller as an Ethereum Request for Comments (ERC). it had been automatically assigned Github issue number 20, giving rise to the name “ERC20 token”. 
• The overwhelming majority of tokens are currently supported the ERC20 standard. ERC20 may be a standard for fungible tokens, meaning that different units of an ERC20 token are interchangeable and haven’t any unique properties. 
• The ERC20 standard defines a standard interface for contracts implementing a token, such any compatible token is often accessed and utilized in an equivalent way.

ERC20 Required functions and events

total Supply:
Returns the entire units of this token that currently exist. ERC20 tokens can have a hard and fast or a variable supply.

balance Of:
Given an address, always returns the token balance of that address.

transfer:
Given an address and amount, transfers that quantity of tokens thereto address, from the balance of the address that executed the transfer.

transfer From:
Given a sender, recipient, and amount transfers tokens from one account to a different. utilized in combination with approval.

approve:
Given a recipient address and amount, authorizes that address to execute several transfers up thereto amount, from the account that issued the approval.

allowance:
Given an owner address and a spender address, must return the remaining amount that the spender is approved to withdraw from the owner.

Transfer:
The event depends upon a successful transfer(call to transfer or transfer From) even for zero-value transfers.

Approval:
Event logged upon a successful call to approve.