is a quaternary ammonium salt
It is clear and colourless, and naturally semi-solid.
It doesn't disinfect against gram negative bacteria, such as the Black Plague or bacteria found in the bowel; or picornaviruses, such as polio, hoof and mouth disease, and the common cold.
One nice thing about benzalkonium chloride (aside from its rockin' name) is that it is purported to inactivate HIV and the herpes simplex virus in vitro. This is not to say that you should play party tricks on this theme.
Where you (or I, for that matter) would most likely get your greedy little paws on some benzalkonium chloride would be in those little antiseptic towlettes that you find in first aid kits. They look like Handi-wipes(tm), but reek of ammonium, and rightfully so.
Typically, these very un-lemony fresh towelettes will contain only 0.01 to 0.4% benzalkonium chloride; anything more and you risk a very, very minor skin burn. More likely, a slight irritation. But don't get it in your eyes. If you do, you'll have to open up E2, access this writeup again, and read this paragraph, wherein I tell you that the best course of action would be to immediately flush your eyes with eye-temperature water for a couple of minutes, and that you really should have done that instead of having spent all that chemical-burningesque time at the computer. Also, don't wear contact lenses when your wiping your eyes with your towelette. Burn. Don't eat it either.
And, yes, yes, there are all those other sensible disclaimers; in any case keep it out of anything that you would describe as part of you that "goes in".
For those who want to heat benzalkonium chloride, it will melt at around 35°C and will boil at around 93°C.
These are other names for the same thing:
Alkyl Dimethylbenzyl Ammonium Chloride; Alkyldimethyl(phenylmethyl) Quaternary Ammonium Chloride; Ammonyx; Arquad DMMCB-75; Barquat MB-50; Bayclean; Benirol; Benzalkonium A; Empigen; Roccal; Zephiran Chloride.