From the cron manpage under Debian Wheezy:
cron also reads /etc/crontab, which is in a slightly different format
(see crontab(5)). In Debian, the content of /etc/crontab is predefined
to run programs under /etc/cron.hourly, /etc/cron.daily,
/etc/cron.weekly and /etc/cron.monthly. This configuration is specific
to Debian, see the note under DEBIAN SPECIFIC below. in order.
"In order" implies (to me) that they are run in sequence. As well, I noticed one or two files on my install are prefixed by numbers, apparently to give them a higher priority and run before others.