C = O
The simplest of the aldehydes, formaldehyde is created through the partial oxidation of methanol. This is one example of a popular method.
      Ag Catalyst
CH3OH -------------> CH2=O + H2
     ~= 650 °C

Annually, over 3.5 billion kg of formaldehyde is produced. Pure formaldehyde, at room temperature, will technically be a gas. In the form that most people will have encountered it, formaldehyde is supplied in an aqueous solution. Specifically, a solution called formalin, which the two writeups on the subject say is a 10% solution, and my textbook says is a 34% solution. Personally, I'm inclined to believe that 10% is closer to the standard strength, but YMMV.

Update: It appears that industrial grade formaldehyde is 34-37%, as is formalin. Formalin, however, is formaldehyde with methanol added, to prevent polymerization. It seems that most of the formalin that you'll encounter in a lab setting, which is where most people would have exposure to it, is 10%. Of course, that isn't to say that all of it does. In fact, it may be that the majority of it does. This is just a rough guideline for what you can expect.

Formaldehyde in its aqueous form is used as a preservative because it is quite effective at killing bacteria. This alone should be enough to show you that you probably want to avoid contact with the stuff as much as possible. As mentioned in formalin, it shouldn't be used for long term preservation, because it is slightly acidic. When exposed to oxygen in the air, it oxidizes into formic acid. As such, you want to keep it sealed up from the air as much as possible.

Drinking a formaldehyde solution is a very bad idea. It will convert to formic acid in your body, and can cause coma, or death. If you, or anyone else is stupid enough to drink formaldehyde, they require immediate freakin' medical attention. I don't know why anyone would want to drink it though, it's not exactly like it smells pleasant.

Most formaldehyde, however, is used in the creation of plastics, particle board, foam insulation, and plywood. It is often used to make adhesives. Specifically, Urea-formaldehyde and phenol formaldehyde resins are used for these purposes quite widely. It is also used in the treatment of textiles, to give them "permanent press qualities".

Products made from these resins, specifically those used in the construction of new homes, can emit formaldehyde gas, which is a bad thing. Formaldehyde has been shown to cause cancer in laboratory animals, and is suspected to be carcinogenic to humans as well. Workers in sawmills which produce particle board also have been shown to have elevated levels of cancer of the nose and throat. In addition, exposure to it in high levels of it can cause or aggravate a number of health problems, including nausea, burning sensations in the eyes and throat, coughing, asthma, skin rashes, and fatigue.

Generally, the rate of emission of formaldehyde gas from one of these resins drops off after time. Warmer temperatures and higher humidity can increase emissions. In addition, Urea-formaldehyde resins will tend to release more formaldehyde than phenol formaldehyde resins.

Molecular Mass: 30.03
Melting Point: -116.9 °C
Boiling Point: -19.0 °C

Material Safety Data Sheets:
Aqueous Formaldehyde, 37%
Formalin, 10%
Formaldehyde Gas

U.S. Environmental Protection Agency, "An Update on Formaldehyde," IAQ Publications. August 26, 2004. <www.epa.gov/iaq/pubs/formald2.html> (November 21, 2004)

Wikipedia, "Formaldehyde," Wikipedia, the free encylopedia. October 4, 2004. <en.wikipedia.org/wiki/Formaldehyde> (Novmber 21, 2004)

U.S. Environmental Protection Agency, "Formaldehyde," Sources of Indoor Pollution. November 18, 2004. <www.epa.gov/iaq/formalde.html> (November 21, 2004)

J. Repka, "Formalin," Everything2.com. May 8, 2002. <everything2.com/index.pl?node_id=1299155> (November 21, 2004)

Hart, Harold, and Leslie Craine, and David Hart. Organic Chemistry. 10th Ed. Bston: Houghton Mifflin Company, 1999.