Definition
A user prompt is the message a user sends to an AI within the rules established by the system prompt. The system prompt sets the role and constraints; the user prompt asks the specific question or assigns the specific task. Effective user prompts include enough context for one task without re-stating rules already in the system prompt.
Example
If system prompt is 'You are a SQL expert', a good user prompt is 'Optimize this query: SELECT * FROM users WHERE...' (not 'You are an expert. Optimize this query...').
When to use
Every conversation turn after the system prompt is set.