From Wiki

Skilled worker migration allows those whose occupation/area of expertise is on the THE LIST to apply for permanent residency.

Permanent residency can be obtained through FSW Category 1 by your occupation/expertise being on the list.

FSW Category 2 is for arranged employment - applying under this category does NOT require your occupation/expertise to be on the list.

There is no longer an FSW Category 3 stream. In its place there is a separate stream under the FSW program called the PhD Eligibility Stream. To quote CIC to apply under this you must:

• be an international student enrolled in a PhD program in Canada (or graduated from a Canadian PhD program within the past 12 months) and meet certain criteria.

There is a limit currently of 1000 applications per year under the PhD eligibility stream. As of May 14, 2012 - this was at 674.

It is recommended that you read the BE Wiki article called Self-Assessment Test-Canada.

As of May 8th 2012, the limit of 10,000 applications under this category for the 2011-2012 intake has been reached. Please Note - this limit does NOT apply to FSW2 applicantions that have an offer of arranged employment.