Vardon grip: little finger of bottom hand fits over index finger of top hand resting between the index finger and the middle finger.
Interlocking grip: little finger of bottom hand interlocks with index finger of top hand going under index finger of top hand and in gap between index finger and middle finger.
Interlocking grip (used by Jack, Tiger and me) is supposedly better for people with smaller hands.
Vardon grip was named after the great Harry Vardon (pictured) who either invented it or popularised it. Up til then most players used the 10 finger grip. He won 6 Open championships. But you knew that.