Do not draw close to a woman when she is a niddah; relations are forbidden. --Leviticus 17:18
Uncleanliness, under Torah law, due to menstruation.
Or almost, anyway. A woman enters niddah when she experiences any uterine bleeding not due to trauma; most often this means menstruation. A woman becomes niddah as soon as blood leaves the uterus and enters the vaginal canal; to test for this, she uses a cloth to check internally for signs of blood. (This examination is called a bedikah; the plural is bedikot).
During niddah, a woman is forbidden by Torah law from a number of actions; Sexual intercourse, any other physical displays of affection such as hugging and kissing, being seen naked by her husband, and approaching the Torah*.
This is contrasted to the state of tehorah (or tahor), in which the woman is clean. A woman must wait until five days into the state of niddah before she can start counting the seven blood-free days (as judged by performing two bedikot each day) that allow her to become tahorah again; after these seven days have passed, she needs to immerse herself in a kosher mikveh (a ritual bath). And she's all set until next time...
Niddah and related rules do not apply during pregnancy.
Needless to say, different traditions of Judaism place different emphasis on these rules. (Some basically reinterpret them out of existence). Check with your local Rabbi. If you want to know more about all of this, http://www.yoatzot.org/category/19 is a pretty good site.
* Heschelian reports that the not touching the Torah rule is "just a minhag of some ashenazi communities - by no authority is it properly forbidden". But e also adds "IANAR!" (and that goes double for me).