Both a manager and mentor will create dialogue, encourage you and correct you. However, while both your manager at work and your mentor "coach" you - there are some differences in the approach.
Your manager will clarify your existing job & accountabilities for you.
Your mentor will explore broad career options.
Your manager will create objectives for your current assignments.
Your mentor will encourage self-reflection and learning goals.
Your manager will teach with a desire to have good performance on their team and for their projects.
Your mentor will ask questions and share personal anecdotes around perceived barriers, challenges & fears with only your development in mind.
Your manager will support personal & career development - usually inside your organisation.
Your mentor will support personal & career development for anywhere you choose.
Your manager will hold you accountable for following direction and achieving your objectives.
Your mentor will cheer you on, while pointing out when you are not being accountable to yourself.
Your manager will assign actions.
Your mentor will suggest actions.
Your manager will listen and escalate issues as they impact the team or project.
Your mentor will listen and keep information as confidential.
Your relationship with your manager is fundamentally one of obligation, driven by your manager.
Your relationship with your mentor is fundamentally one of choice, driven by you.