Let me tell you how it works - based on my experience, can be different now, do your research

There are 2 programs to migrate to Canada as a Permanent Resident under skilled workers. One is Federal (can apply without job offer) and another one is Provincial. 

Under Federal program, they have certain criteria (that changes every year) you need to match in order to apply for PR. The basic are

• PR  points (at my time it was 67). It is multiple sets of questions that ask your education, age, work experience etc. If you score atleast 67 you are eligible for next round. You can google "Canada PR calculator" or here is one for you http://www.canada-da.com/calculator.html
• Next go through the eligible occupation list (I have pasted in this page), to check whether your experience/occupation matches with it. Also they change this list every year. 
• Than you apply with all the documents in the checklist. You need to show documents related to work experience/birth certificates/transcripts/IELTS score etc

Under provincial program, you need to have a job offer. 

Eligibility Criteria:

http://www.cic.gc.ca/english/immigrate/skilled/apply-who.asp

You must have one year of continuous, full-time (or an equal amount of part-time) paid work experience, in at least one of these occupations within the last ten years:

• 0211   Engineering managers
• 1112   Financial and investment analysts
• 2113   Geoscientists and oceanographers
• 2131   Civil engineers
• 2132   Mechanical engineers
• 2134   Chemical engineers
• 2143   Mining engineers
• 2145   Petroleum engineers
• 2144   Geological engineers
• 2146   Aerospace engineers
• 2147   Computer engineers (except software engineers/designers)
• 2154   Land surveyors
• 2174   Computer programmers and interactive media developers
• 2243   Industrial instrument technicians and mechanics
• 2263   Inspectors in public and environmental health and occupational health and safety
• 3141   Audiologists and speech-language pathologists
• 3142   Physiotherapists
• 3143   Occupational Therapists
• 3211   Medical laboratory technologists
• 3212   Medical laboratory technicians and pathologists' assistants
• 3214   Respiratory therapists, clinical perfusionists and cardiopulmonary technologists
• 3215   Medical Radiation Technologists
• 3216   Medical Sonographers
• 3217   Cardiology technologists and electrophysiological diagnostic technologists

Goodluck