5K, 10K, half, full
5K and 10K mean five and ten kilometres. A half marathon is 21.0975 km, usually shown as 21.1K. A marathon is 42.195 km. Event names do not always tell you every distance offered.
Pace versus speed
Pace is usually minutes per kilometre. A 6:00 min/km pace means each kilometre takes six minutes. Speed measures distance per hour instead. Check which unit a group uses before comparing numbers.
PB, chip time, gun time
PB means personal best. Chip time generally measures from crossing the start mat to crossing the finish mat; gun time begins with the race start signal. The event rules determine which result counts for awards.
Social run, long run, no-drop
A social run emphasises company. A long run is relative to your own training. “No-drop” usually means the group keeps everyone together or regroups, but always ask how that works in practice.