> If I have a 6 in the hour field of a crontab entry, does that mean the job
> will run only at 6am or will it run at 4pm too (16:00)?
> 
> I'm thinking 6am only. Do I understand this correctly?

if you put just a 6 in the hour field, then it will only run at 6am.  If you 
want it to run at more than that hour, then you would do something like:

* 6,14 * * * command <--- runs at 6am and 2pm
* */6 * * * command  <--- runs every 6 hours starting at midnight