An Altcoin is any coin which isn't Bitcoin. Since Bitcoin was created in 2009 there have been many other Altcoin. So any crypto coin that in not Bitcoin is known as an Altcoin.
Take the word “Alternative”, shorten to “Alt”. Then take Cryptocurrency and replace with the simpler “Coin”. “Altcoin” is a combination of two words: “alt” and “coin”. Thus together they imply a category of cryptocurrency that is alternative to the digital currency Bitcoin
Alot of the Altcoins are basically copies of the original Bitcoin. They use the same “blockchain technology” and bring nothing new.
Not all of the altcoin are copies of Bitcoin
Some have copied the technology and tweaked it. Others changed parts of it to make new different features available in the new currency.
Some of the Altcoins have been built from scratch, using different blockchain technology and each has its own merits. Therefore it's worth doing some research first before dabbling in any new Altcoin.
While many altcoin promise useful features, upon closer inspection many of these promises are just that: think politicians.
Many altcoins can be considered outright scams, mainly created to enrich its inventors and early adopters.
There are new coins being created all the time. So many so that at the time of writing this there are over 1,500 different crypto currencies, although only about 100 are actively used by the public to the degree to be notable.
The main Altcoins that are in regular use, liquid and popular are Litcoin, Dogecoin, Ethereum and Ripple. Litecoin was branding Silver to Bitcoin being Gold, but really Ethereum should have that mantle.
Altcoins are typically riskier than Bitcoin. Their exchange rates are often more volatile, and over the years virtually no altcoins have maintained their value against bitcoin.
Their are also some coins which are no longer used, essentially digital dust.