Altcoins are cryptocurrencies other than Bitcoin. The majority of altcoins are forks of Bitcoin with small uninteresting changes. This page categorises different ways altcoins have modified Bitcoin.
Many people prefer the term "shitcoin" to clearly distinguish all these altcoins from Bitcoin. This term has even been used in US Congress in 2019.
The two main innovations in Bitcoin are its digital scarcity and its censorship resistance. For every altcoin you can point out that 1. it's trying to copy Bitcoin and that 2. there's a clear person or group of people in charge. Thus it's very easy for someone else to copy an altcoin and try to push it with even more marketing efforts than the "original" altcoin. Additionally it's relatively easy for government actors to interfere with the workings of an altcoin (such as Ethereum) by simply knocking on the doors of whoever is behind the altcoin.
Some people are able to make money when trading altcoins, but for most people it's much preferred to simply buy and hold bitcoin for the long run. A lot of people would have been much better off with this simple strategy. Many people think that they make money, but if you look at their total net worth in Bitcoin terms they lost both money and time by trading on so called shitcoin casinos.
