Each user has their own crontab, and commands in any given crontab will be executed as the user who owns the crontab. Blank lines and leading spaces and tabs are ignored. Note that comments are not allowed on the same line as cron commands, since they will be taken to be part of the command. Similarly, comments are not allowed on the same line as environment variable settings. An active line in a crontab will be either an environment setting or a cron command.
Otherwise mail is sent to the owner of the crontab.
The format of a cron command is very much the V7 standard, with a number of upward-compatible extensions. Each line has five time and date fields, followed by a user name if this is the system crontab file, followed by a command. Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive.