If the lines are on the negatives themselves, the damage is usually either from the camera/film path or from processing.

A good clue is whether the marks run perfectly parallel to the film edges:

• Parallel lines often point to dirt or a small particle in the camera or film cassette scratching the film as it advances.
• Irregular or non-parallel lines are more likely caused during processing or drying.

Based on the descriptions in the answers, these marks are very likely lab-related—either poor maintenance in a roller-transport processing machine, or squeegee/chamois marks from manual or semi-manual processing after the final rinse.

To reduce the chance of it happening again:

• Inspect and clean the camera film path if you suspect debris.
• Try another lab, especially one with a good reputation for film handling and machine maintenance.
• If using a small lab, ask how they process and dry film.

Poor loading or rewinding is less likely to cause long continuous scratch-like lines of this type than debris or processing damage.