can someone tell me what is difference between background check and Security check?
A security check consists of ascertaining whether or not an applicant has a criminal record, is likely to be a terrorist threat, etc. I am guessing that the visa officer who reviews an applicant's file looks at the police clearance certificate(s) he/she has submitted. But assessing the security risk that an applicant may or may not pose to Canada may involve more than just reading police clearance certificates. I don't know what else the visa officer does to fulfill his/her duties in this regard.
When a visa officer does a background check, he/she satisfies him/herself that the statements that the applicant has made about him/herself (name, family members, employment history, etc.) are true.
Although I've called them separate processes, I imagine that background and security checks sometimes may be related to each other. For example, I can visualize a scenario in which the visa officer discovers something odd while he/she is doing the background check, and that motivates him/her to do a more thorough security check. That is just a hunch on my part, but it makes sense to me.
Read this document too
https://fas.org/asmp/campaigns/control/Canadianexemptionsfoia/FOIA_4_22_05/P5HX.pdf
Information collected from other sources:
http://britishexpats.com/forum/immigration-citizenship-canada-33/difference-between-security-check-background-check-499467/