Cron will be started next time you reboot. Vixie Cron will automatically reload the files after they have been edited with the crontab command. Some cron versions reload the files every minute, and some require restarting, but Vixie Cron just loads the files if they have changed. If you place a script into one of those directories it will be run either hourly, daily, weekly or monthly, depending on the name of the directory. If no one if specified, then the output will be mailed to the owner of the process that produced the output.
HOME is the home directory that is used for cron.
Now for the more complicated second part of a crontab file. There are normally seven fields in one entry. This is the Day of Month, that you want the command run on, e.g. to run a command on the 19th of each month, the dom would be 19. This is the user who runs the command. This is the command that you want run.