The answers given in the question linked to in AndrewH’s comment are correct.
As noted on this Photoshop help page:1
Tracking and kerning are both measured in 1/1000 em, a unit of measure that is relative to the current type size. In a 6‑point font, 1 em equals 6 points; in a 10‑point font, 1 em equals 10 points. Kerning and tracking are strictly proportional to the current type size.
In other words, you can achieve the same CSS letter-spacing by dividing the Photoshop tracking value by 1,000 and using the quotient, in ems, as the letter-spacing value.
1 The page linked to is for Photoshop, but the same holds true for all other Adobe products. According to Wikipedia, Quark has 1/200 em as its base unit.