When the limited up/down of a futures is triggered, is trading still going on but the price cannot move further? Is it only common in the commodities?
Yes, the market is still open (unless there is a trading halt, like there could be in the stock indices). Trading can still happen at the limit price but usually the volume comes to a screeching halt. For example, if the fair price for a barrel of oil is $20.00, but the NYMEX market is limit up at $19.50, then you'll have lots of people bidding $19.50 but no one offering $19.50 and virtually no trading. By commodities do you mean physical commodities as opposed to financial futures? Financial futures have limits also. See the exchange websites for particulars.