Bitcoin is the first cryptocurrency created in 2009 by a person or group of people under the name of Satoshi Nakamoto. Bitcoin was proposed as an alternative to the conventional banking system in the wake of the financial crisis of 2008. Unlike banks, the Bitcoin network is a decentralized anonymous peer-to-peer payment system designed to the way that it is almost impossible for a single entity to gain control over this network.
Tether is a cryptocurrency launched in 2014. It is a so-called stablecoin currently leading by market capitalization. Although according to the initial idea, each Tether was backed by $1 (that was going to keep the price of Tether stable), the company not always managed to maintain this price (at some point in October 2018 the price dropped to $0.9 per one Tether). Tether succeeds in terms of market capitalization and its price is almost all the time is really close to $1.