Glossary
Fact witness
A witness who testifies about what they personally observed, did, or experienced. Most witnesses are fact witnesses. They can't give opinions on technical matters: that's the role of an expert witness.
If you saw a car crash and you're called to testify about what happened, you're a fact witness. You can describe what you saw, what you heard, where you were standing, and how fast the cars seemed to be going (using ordinary judgment), but you can't testify that the collision caused a specific medical injury: that needs a medical expert.
Fact witnesses are usually unpaid (beyond a small witness fee for the day), don't need any special qualifications, and don't usually need to write a report. They're called because they happen to know something relevant.