If you are applying for PR, you can get extra points of you have a job offer. That is when you need an AEO - Arranged Employment Opinon from HRSDC (Human Resourance and Social Development Canada).
LMO - Labour Market Opinion is needed when an employer wants to hire a foreigner on a temporary work permit. It's basically looking at the job market if there really is a shortage of people in that field, has the employer advertised for people locally, is he offering competitive wages in that industry etc. Employer can either apply for an LMO for a specific person he wants to hire or in many cases big companies will apply for several LMO's at once in their field so they can hire foreign workers faster as they find somebody who is qualified. The foreign worker will still need to apply for a work permit but the positive LMO is needed first.
http://www.hrsdc.gc.ca/en/workplaceskills/foreign_workers/supperimm.shtml
