What does the term Postdoctoral mean?
In its simplest sense, 'Postdoctoral' is an adjective used to describe any area in academia or research that relates to someone who has completed a doctorate (PhD).  In this respect it is very broad, and something of a catch-all term, including not only junior academics, but in reality all those who holds doctorates.

In everyday academic and research usage, however, 'Postdoctoral' applies to the work and position of a junior academic who is conducting specialized research and is not in a permanent post.  A component of this research is further specialized training and the acquisition of experience in fields that might or might not be directly related to their doctoral work.  For many postdoctoral fellows, those individuals holding postdoctoral positions or 'fellowships', this is a period where they can carve out their own niche in a research field, or explore and gain experience in another field, allowing them to complement the work they conducted during their doctorate with that of another field. Postdoctoral fellows then are primarily concerned with doing research and gaining training in that research in varying degrees.

Postdocs and Research
- cutting edge research, pushing boundaries (43% of Science papers are lead-authoured by a post doc)
- typically 10+ years of training (B.Sc. or a B.A. + Ph.D.), often accompanied by significant student debt

Because postdoctoral fellows are highly trained specialists and are primarily focused on research, they occupy a special place in academia. Even more than doctoral students, postdocs frequently push the boundaries of innovation. As young scholars, their work is on the cutting edge - they have received at least a decade of training, but stand on the very start of their scholarly careers. Often the most innovative moments appear in the space in between doctoral work and a permanent position. The pressure to carve out their own work and identity makes postdocs gamblers of a sort - those whose very job is to ask new questions of old research, or blaze new trails. When they finally land a permanent position it is because their work is at the frontiers of their field, and in many cases they will spend the next few years exploring the impact of the discoveries and research they've conducted during their fellowships.

Who are Postdoctoral Fellows?
- hold doctorates (usually PhD)
- junior academics (~20% of whom will become professors, depending on the field)

As mentioned above, PDFs are those who hold doctorates, but typically those who hold a fellowship or funding to promote research in between the completion of their doctorates and the obtaining of a permanent position - either in private industry, the public sector, or as an academic.

Confusion may arise when the term "postdoctoral fellow" is occasionally also used to refer to post-residency medical doctors engaging in a particular clinical fellowship. These individuals are in a very different situation in terms of role (direct patient care) and remuneration (2-fold higher or more) than the "academic" or "research" postdoctoral fellows, to whom this term usually refers, and the distinction between the two is therefore important. It is important to note that, as compared to PDFs, post-residency medical doctors receive no research training and do not have the same expert research skills as PDFs. This is often a source of misunderstanding among politicians and the general public.

How many are there in Canada?
~6000
Formal numbers do not exist because many institutions do not formally track their PDFs

How are Postdoctoral Fellows funded?
- Two channels:
a) by a direct, personal fellowship (very flexible yet can only be obtained during first 2-3 years)
b) under a project grant held by a professor (typically for a particular project, less flexible)

- PDF fellowship is between $35,000-45,000 (2009 CAPS survey of Postdocs; 59% of respondents)
- generally work significantly more than a "standard" 40 hour week
- often no other funds available aside from grants/fellowships (i.e.: can't work overtime, no hourly pay, other work requires a more senior position)
- teaching roles sometimes provide additional funding (However, many external fellowships specify that a PDF is not allowed to have any additional sources of income)
- postdocs have both a research and a training component
- mentorship and teaching roles

Generally, PDFs receive funding from one of two channels - direct funding from a research agency, a university, NGO, private corporation, or government department; or indirectly via a research project, centre, or lab who receives research grants from the same sources. Those who receive direct funding often have more control over their research, while those who receive their fellowships from projects, centres, or labs, work under the supervision of a 'Principal Investigator' who directs the collaborative research efforts of a number of junior academics - including PDFs, but also doctoral students, and others.

These differences in funding mean that there is considerable flexibility in what a postdoc can do - some work traditional hours of 9-5, but usually much more, while others are left to their own devices. Because research is the primary objective, teaching is limited, but nonetheless they receive on-the-job training from others, but also are informal instructors for those they supervise and interact with in research contexts.  
Some are able to supplement their income by teaching the odd university level course, but predominantly the Postdoc is an applied position where research is the main component.