Ruach HaKodesh is a Hebrew term:
Ruach (pronounced Roo-akh) - Wind, Spirit, Soul.
Ha - literally 'the', functions as 'of'1
Kodesh - Holyness, sanctity
In Jewish thought, Ruach HaKodesh refers to a particular type of prophecy, traditionally the type that the Patriarchs received. It isn't what might be considered 'true prophecy' though.
While not clearly defined, it's clear that Ruach HaKodesh refers to a sort of Divine inspiration. The Midrash often refers to people as having seen something through Ruach HaKodesh, and much of the Old Testament is said to have been written through Ruach HaKodesh (But not the Torah -- Orthodox Jewish belief says that was written a much higher level of revelation).
It's unclear exactly how Ruach HaKodesh functions. The sources seem to make it some kind of inspired hunch -- like the Jargon File's zen. There's also a reference in the literature to Jacob not being able to 'use' Ruach HaKodesh while in mourning for Joseph, because it requires the user to be at peace.
At least one
literal translation of Ruach HaKodesh is
"the Holy Spirit", and when used in
Christian theology it means just that. In fact, do an Internet search on "Ruach HaKodesh" and you'll mostly find
Messianic 'Jewish' websites using the term to refer to
the Holy Ghost in a Jewish-sounding way.
1As Gritchka has pointed out, this isn't exactly true. However, it'll do without explaining Hebrew construct forms
http://www.shemayisrael.co.il/dafyomi2/megilah/points/mg-ps-07.htm
http://www.ohr.org.il/misc/eterni-7.htm
http://shamash.org/listarchives/mail-jewish/volume13/v13n39