Altcoins are tokens created on a blockchain which is not the original Bitcoin. These could simply be a copy or for of the original Bitcoin or it could be a completely different blockchain with different properties and purpose.
Bitcoin is still the King of Crypto’s in terms of market cap and dollar value. In order for an alternative to beat Bitcoin it will need to revolutionise the crypto industry which itself is a challenger to the status quo. The most interesting altcoins are ones that build on top of other protocols as it brings bigger communities to its support base, which result in higher token value over the long run.
Ethereum give incredible utility to holders of its ETH cryptocurrency which is an altcoin. This utility drives up the value of the token. Whilst it is second only to Bitcoin, it has struggled to deliver on the delivery of Ethereum 2.0 which is promised to be a PoS based protocol.
You can trade altcoins almost everywhere you can Bitcoin. Furthermore you can trade or earn these tokens on centralised or decentralised platforms. Trading volume and chain analysis suggest that Binance is the most prolific centralised exchange. Its selection of altcoin, bitcoin and FIAT trading pairs more liquid than most other venues. DEX trading platforms like Uniswap have come to prominence lately. However, the high cost of GAS fees on the Ethereum network has hampered growth.
NFTs are technically not Altcoins because they are by definition non-fungible. However, we can see them as alternative crypto assets because they represent digital media which may have value. You can also trade them. Ultimately you tie your ownership of an NFT to a crypto address on a blockchain.
DeFi protocols such as Aave allow participants to earn interest in the form of AAVE (aka LEND) tokens. Uniswap is a very popular DEX that allows users to swap almost any ERC20 token pair without any 3rd party. Liquidity providers must stake both sides of the liquidity pool in a 50/50 ratio. In return they earn a proportion of transaction fees as well as the UNI governance token.