Answer:

Some examples are better than thousands of theoretic words. : )

Example 1:
The cron expression of your cron job is * * * * *,
Your cron job will be executed every minute.

Example 2:
If the cron expression is */8 * * * *,
Your cron job will be executed every 8 minutes in every hour. Please remember that the time calculating is not accumulative. In this example, if you create the cron job at 23:59:00, your cron job will be run at (theoretically, the actual run time could have some delay, same for below examples):
00:00:00
00:08:00
00:16:00
00:24:00
00:32:00
00:40:00
00:48:00
00:56:00
01:00:00
01:08:00

Your cron job will not be executed at 01:04:00, because another hour has started here and the time is not accumulative.

Example 3:
If the cron expression is */8 */3 * * *,
and you create the cron job at 23:59:00, your cron job will be run at:
00:00:00
00:08:00
00:16:00
00:24:00
00:32:00
00:40:00
00:48:00
00:56:00
03:00:00
03:08:00

Your cron job will not be executed at 01:08:00 anymore, because */3 means the hour number should be divisible by 3.

Example 4:
If the cron expression is 3 5 * * *,
your cron job will be run at 05:03:00 everyday.

There is some more advanced examples at our Cron Expression page.