Casual conversation blurs these three terms. Specific usage in the DnB community is more precise.

Track

A single piece of music. The 5-minute file the producer made and released. The smallest unit of DnB consumption.

Mix

A recording of a DJ playing multiple tracks in sequence with transitions between them. Usually 30-60 minutes long. Posted on Soundcloud, downloaded, listened to as a continuous experience.

Set

A live performance by a DJ. The mix recording captures a set; the set itself is the act of playing in a venue. "I caught her set" means you were at the live performance. "I downloaded her mix" means you listened to a recorded one.