Bitcoin Forum

For what purpose does etherdelta.com website is used
Is it for exchange like polo, bitfinex, bittrex

Yes you are right. Etherdelta is a fully decentralized trading platform.
This means you don't have to trust an administrator to hold your funds. Your funds are held in a smart contract in such a way that you and only you can deposit, withdraw or trade.

Yeah, it's an exchange. But it's decentralized, meaning there is no central server. It's all done peer-to-peer (p2p)

Keep in mind that EtherDelta only have pairs between ERC20 tokens from the Ethereum network. If you want to trade BTC/LTC/DASH etc, you still need to use a centralized exchange such as Bittrex and Poloniex.

Can we exchange our erc20 tokens in etherdelta for some valuable coins such as eth, btc, ltc ???

Yes it is an exchange but there is a catch that only the tokens that have been created on the Ethereum platform i.e., ERC20 tokens alone can be exchanged on this website. This website doesn't accept BTC deposits and withdrawals. It functions only on ETH coins. Most of the tokens are first available to exchange on etherdelta and then on other exchanges. We need to have at least the minimum gas limit to start a trade and that's the only thing I dont like about it.

No, you can't.
Your question has answered above.
if you want to exchange erc20 tokens for eth, btc or ltc, you may try bittrex, poloniex, binance bitfinex or bithumb.
However, I would recommend to use a local exchange in your country as you may find it easier to verify your Id in case it's required to withdraw cryptocurrency, but in most case, no need to verify Id to withdraw cryptocurrencies, unless you want to withdraw fiat currency.