USDT is a digital currency that is stable, pegged to the US dollar. People use it at times to hold their profits when they trade other digital currencies that fluctuate. You find USDT on popular exchanges like bittrex as direct pairs against BTC, ETH and other digital currencies. You can learn alot about this even passively by being here and consuming content and via your approach, which is 'doing this and that on bittrex'. On coinmarketcap, you can pick whichever coin you want and visit their websites directly to read about them. Some or most of these digital currency have an underlying blockchain and other utilities besides just trading or 'financial services'. For instance, STEEM currency is backed by a blockchain called steem, which has built on it several applications. STEEM has several utilities, e.g you may want to buy it to power up and have influence on the social platform called steemit. you may need steem for resource credits, you may need it to power up to claim steem accounts to run a service that allows instant sign up, one may buy it to pay for services on apps that only accept steem etc. This applies too to many of the digital currencies you see out there.