keywords volatile in an interrupt routine

In this webpage :http://www.keil.com/support/man/docs/gsac/gsac_timer.htm
The question is why two variables one use volatile but another not ??
The two variables is "TimeTick" and "clock_1s".
Who can tell me why,i am very confused. thank you !

/*----------------------------------------------------------------------------- * Name: Timer.c * Purpose: Timer example. Prints a sting to UART2 * Notes:
*----------------------------------------------------------------------------*/
#include <stdio.h>
#include <stm32f10x_cl.h>

volatile unsigned long TimeTick; // Counts 10ms timeTicks
unsigned char clock_1s; // Flag activated each second

extern void init_serial (void); // Function defined in Serial.c

/****************************************************************************** Initialise the TIM3 for 1ms @ 72MHz
******************************************************************************/
void TIM3_Init (void) {

RCC->APB1ENR |= (1<<1); // enable clock for TIM3

TIM3->PSC = 3; // set prescaler TIM3->ARR = (18000000UL / 1000UL) - 1UL; // set auto-reload TIM3->CR1 = 0; // reset command register 1 TIM3->CR2 = 0; // reset command register 2 TIM3->DIER = (1<<0); // Update interrupt enabled TIM3->CR1 |= (1<<0); // Enable Timer

NVIC_EnableIRQ (TIM3_IRQn); // Enable TIM3 interrupt
}

/*----------------------------------------------------------------------------- Timer Counter 3 interrupt service function executes each 1ms @ 25 MHz Crystal Clock
*----------------------------------------------------------------------------*/
void TIM3_IRQHandler (void) {

if (TIM3->SR & (1<<0)) { // UIF interrupt? if (TimeTick++ >= 999) { // Set clock_1s to 1 every 1 second TimeTick = 0; clock_1s = 1; } TIM3->SR &= ~(1<<0); // clear UIF flag }
}