The immigration officers need to know if you have a criminal record. Therefore, they'll ask you to provide Police clearance for all the countries where you have lived more 6 consecutive months.
I suggest you to visit this Canadian web link for further details on this matter :
http://www.cic.gc.ca/english/informatio ... /intro.asp
As soon as you have copies of police certificates of your home country and any other countries where you have lived, you should send them to the Canadian immigration officers. Upon reception of these documentations, they'll continue reviewing your application and determine your admissibility into Canada. The Canadian Immigration may transfer these information to the RCMP (Federal Police) or any other Canadian departments for further checks and sometimes compare information with foreign police service (possibly with Police bureau in your home country or the USA or Interpol).
It's better not to hide any information as it may be either delay any answer until all checks are done (could be months, or even years) or a reason to deny entry into Canada, therefore a PR visa , work or tourist visa.
