An altcoin is any cryptocurrency that is not bitcoin, or a stablecoin (one whose price tied to a real commodity like US dollars or precious metals). This includes all the big names like Ethereum, Ripple, Bitcoin Cash, Litecoin, and thousands more.

Altcoins range in trading volume and market cap. Several are in the billions of dollars of market cap, hundreds are millions, but the majority are tiny with barely any money in them, and rare trades.

Investing in altcoins is essentially like investing in the underlying company like a venture capitalist might. You're betting that they'll become successful.

If they take off, you could end up with a 1000x return. But most will go nowhere.