Jiffies : The jiffies variable holds the number of times the system timer popped since the system booted.
HZ: The number of timer ticks per second, is contained in the kernel variable HZ.
The kernel increments jiffies, HZ times every second
Jiffy: On a kernel with a HZ value of 100, a “jiffy” is a 10-millisecond duration, while on a kernel with HZ set to 1000, a jiffy is only 1-millisecond.
For more info
http://www.linux-mag.com/id/2272 (The Passage of Time)