Think of someone vocally (because that's often how people type --- as though they're speaking) telling you a series of arithmetic:
One plus two times three minus four plus three times two.
To vocally say that, in the above order, with proper math, it would probably be something like:
One, plus the result of two times six, minus four plus the result of four times two.
One + (2 * 3) - (4+(3 * 2)) ("One; plus two times three; minus four plus 3 * 2")
Or (One + (2 * 3) - (4+3) * 2) ("One; plus two times three, minus four plus three;  times two).
But in reality, even the times two is still quite vague, even being generous with taking pauses as groupings. That's why people say things like "... all times two." or more importantly why we rarely do math purely vocally and why we have "The quantity [...] plus the quantity [...]" etc.
edit: HN, it's 2012. Frigging implement Markdown and get it over with.