A rule for arranging chat messages in the format a chat model expects.
Same words, very different vibe: lunchbox note or detention slip. A chat template is the little form with all the boxes.
It puts the system message and user message in the right spots. Different LLMs expect different forms. This can change answers and compatibility.
Prompt
A chat template decides how the prompt becomes chat input.
System prompt
A chat template decides which message spot holds the system prompt.
LLM
Different LLMs often use different chat templates.