What is Government security clearance?

[arcticmaple]

Dear Seniors,
                  Can anyone tell me what is Government security clearance. Actually I applied for IT job in Canada and the Canadian Employer wrote me back asking do i have Government security clearance. Now what does this mean since i got my PR visa 2-months back(not yet landed), do i again have to apply for Government security clearance in Canada to seek employment there? I have to reply back to the employer very soon. Looking for answer to write back to him.
Please, can anybody help me with this.
Thanks & Regards,
arcticmaple...

[Baloo]

What specific level are they asking for?

http://immipedia.ca/Security_Clearance_(for_jobs)

I have some more updates to add to the linked page, when I get some free time.

[arcticmaple]

Dear Baloo, Thanks a lot for you prompt reply. Where can i get the Security Clearance done? Can i do it here in India or have to do it in Canada? If i have to do again then what is the use of getting PR if we cannot apply for even non gov jobs. I have not applied for any govt jobs and i don't understand why they require Security Clearance from me? I just got my PR visa 2-months back. I thought if a person gets his PR visa, he is eligible to work anywhere in canada except GOVT Jobs which require citizenship. So every time i apply for any kind of job do i need to do security clearance?
Thanks & Regards,
arcticmaple.

[Baloo]

Obviously, if you need clearance for a private company, the checks may differ.
For government work, job agencies are the usual place to get the clearance (they will do it for you if you are likely to get a job and meet the requirements).
You cannot apply for clearance yourself.

I can only speak in general terms here, every case is different.
In most cases the following applies. But in some cases the terms are varied to allow certain applicants a clearance.

ER status
Normally requires the person to have resided in Canada for 5 years (living and working).
Expires after 10 years.
Can take 2 days to 4 weeks avg - fingerprints 8 - 18 weeks - face to face interviews 4 weeks - Out of country checks 50+ weeks.

Confidential and Secret
Normally requires the person to have resided in Canada for 10 years (living and working).
Expires after 10 years.
1 to 6 months avg  - fingerprints 8 - 18 weeks - face to face interviews 4 weeks - Out of country checks 50+ weeks.

Top Secret
Normally requires the person to have resided in Canada for 10 years (living and working).
Expires after 5 years.
Up to 2 years - very intrusive, neighbours, friends and associates can all be interviewed.

Security clearances / checks are a chicken and egg situation, if you do not meet the pre-conditions they can be hard to gain.

A PR can work in jobs that do not require clearance, security mandated jobs are a different matter.
If you have a specialised skill, agencies are likely to apply for your clearance, if you are extremely specialised there are other routes available to the government.
e.g if you have skill that CSIS or maybe DFAIT needs, they can obviously request a modification of the rules to enable your placement.