ERC20 is a standard describing functions and events each and every token smart contract should implement for compatibility and interoperability purposes. This is necessary, otherwise wallets and other software would have to add custom independent support for every new token created. You can read more here:
https://github.com/ethereum/EIPs/issues/20.
I am bit confused between token and coin. For an instance i was offer ed free few $FUCK tokens but i wrongly gave my ETH address of my JAXX wallet. i couldnt find my coins there?
I don't think token has any formal definition, but it's most often used to refer to assets created by smart contracts within some blockchain ecosystem. For example, FUCKTOKEN is a token created by the smart contract deployed on Ethereum network. You therefore need Ethereaum wallet to store and use FUCKTOKEN. That said, your wallet program has to know the address of the smart contract used to create a token (and some other things about a token such as its name and decimal places) in order to able to show your balance and use that token for future transaction. Not sure, how JAXX handles all of this as I don't use it.