Anthropic's model sizes, named small to large. Haiku is fast and cheap, Sonnet is the balanced middle, Opus is the most capable and most expensive. Other companies have the same ladder under different names (GPT-mini vs GPT, Gemini Flash vs Pro).
Why it matters: a common cost rule is "cheapest model first": start every task on the small tier and only escalate when it fails. Ten Haiku calls can cost less than one Opus call.