1. For changing the runlevel you should have to go in
/etc/inittab. There you can change the runlevel what you
want.It will not affecting current user. After restarting
system your system will boot in which run level you have set. 

3. Internel command means whichever command are builtin the
system BIOS. And External command means Which are
nonbuiltin.It outside the shell. It require shell.

4. VMstat & sar are showing the free memory of the system.
5. ls command is use list a file & cat & vi is to open a file.
6.

1) Same answer as above

2) Question not understood, may be ACLs

3) It is not BIOS, 
A built-in (internal/resident) command is one that is
contained within the bash tool set and execute faster than
external commands.

An non-built-in (external/non-resident) command is a command
outside the shell and requires a $PATH (environment
variable) to find it.  

4)Displays the activity for the CPU

# sar
Linux 2.6.18-6-686 (server.domain.local)       09/16/2008
11:31:56 AM       LINUX RESTART

5) # lsof

6)The Linux kernel is a complex program which provides the
underlying services to the rest of a Linux distribution. But
it is easy to add new features or improvements to it,
requiring a kernel recompiling. There are three reasons for
a kernel compile. Firstly, you may have some hardware that
is so new that there's no kernel module for it in on your
distribution CD. Secondly, you may have come across some
kind of bug which is fixed in a revision of the operating
system. Lastly, you may have some new software which
requires a newer version of the operating system. 

7)A vague question

8) Suppose we have a daily update script called dailyupdate.sh
Copy the script to /etc/cron.daily/

# cp dailyupdate.script /etc/cron.daily/

Make the file executable

# chmod +x /etc/cron.daily/dailyupdat.sh

9) COM1

10) Depends...

4) Detailed sar output

Linux 2.6.18-6-686 (server.domain.local)       09/16/2008

11:31:56 AM       LINUX RESTART

11:35:01 AM       CPU     %user     %nice   %system  
%iowait    %steal     %idle
11:45:01 AM       all      0.71      0.00      0.19     
0.18      0.00     98.92
11:55:02 AM       all      1.53      0.00      0.19     
0.26      0.00     98.02
Average:          all      1.12      0.00      0.19     
0.22      0.00     98.47

1. use init <runlevel>, at runtime.
2. disk quota has been set and reached for the current user.
3. same as above
4. same as above
5. same as above
6. lay man definition, adding support for required hardware
and modules in the Linux kernel.
7. sysreport, question too generalized, 
8. same as above
9. same as above
10. :)