New investors often lump "crypto" into one bucket, but the assets inside it are built for very different purposes.
Bitcoin
Bitcoin was designed to be digital scarce money — a fixed supply of 21 million coins, no central issuer, and a track record going back to 2009. Most people who hold it long-term treat it more like digital gold than a currency they spend day-to-day.
Altcoins
"Altcoin" just means any coin that isn't Bitcoin. Some, like Ethereum, are platforms other applications are built on top of. Others are speculative projects with much smaller track records, weaker security assumptions, and far higher volatility. Altcoins as a group are riskier and more prone to going to zero than Bitcoin has historically been.
Stablecoins
Stablecoins (like USDC or USDT) are designed to hold a steady value, usually pegged 1:1 to the US dollar, backed by reserves. They're useful as a "cash" position inside crypto — a way to hold value or move funds between exchanges without going back to a bank — but they carry their own risks: the issuer's reserves and redemption process are only as trustworthy as the company running them.
Knowing which bucket an asset falls into changes what questions you should be asking before you put money into it.