Any cryptocurrency other than Bitcoin is technically an altcoin - a category that spans everything from Ethereum (a trillion-dollar smart contract platform) to a three-day-old dog-themed token with a $50,000 market cap. The range of quality, purpose, and risk within this category is enormous, which is why treating all altcoins as a homogeneous group leads to poor investment decisions. A useful mental framework: large-cap altcoins like ETH, SOL, and BNB have proven products, large user bases, and real developer activity. Mid-cap alts carry more speculation premium. Small caps are essentially venture bets where most will fail and a handful will generate outsized returns.