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There are several different things called Schedulers in Linux (the Kernel) and in the GNU applications, system, and utilities. At what level are you directing your question? Realize, if this is homework you are likely to get only a link to help you do the research you should have done before asking.
@arun5002: Excuse me, cron and at are only applications. You can install Linux without them, and low-level process scheduling still takes place.
'cron' or 'at' can be installed and used with linux (or AIX, or HP-UX, or Solaris, etc) but they are options. You can install any Linux distribution without them, and Linux cannot be said to run on something that it runs just fine without. The only ones you can really say "linux runs on" are the ones internal to the kernel.
Application level is only one of several places where there are schedulers of different kinds. We still do not know the intent of the OP's question. It appears by the delay in receiving feedback from the OP that he has no intention of refining his question or providing feedback about his intent.
# Edit this file to introduce tasks to be run by cron.
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
# For more information see the manual pages of crontab(5) and cron(8)
# m h dom mon dow command
@reboot echo "Reboot"
0 7 * * * sh myscript.sh
I think the OP is probably asking about low-level task scheduling (but I could be very wrong).
If that's right, macemonta's answer is relevant, but not the only option (see also BFS, for example).
Of course, there is also an I/O scheduler, and there options there, too.
There are also, as has been mentioned, mechanisms for running tasks at particular times, but I don't think that is what the thread was intended to be about. We won't really find out what the OP wanted to know unless the OP makes some more contribution to this thread, and that currently looks as if it might not happen...