Answer:
   
   Based on the words in Deut. 12:3-4, the Rabbis deduced that it is
   forbidden to erase the name of G-d from a written document. Since any
   paper upon which G-d's name was written might be discarded and thus
   "erased", the Rabbis forbade explicitly writing the name of G-d,
   except in Holy Books, with provisions for the proper disposal of such
   books.
   
   According to Jewish Folklore, G-d has 70 names. However, only one of
   these names is the ineffable name, which cannot be erased or
   pronounced. Further, of the 70 names, seven may not be erased but they
   can be pronounced on certain occasions (such as when reading the
   Torah). The other names may be erased and pronounced, but still must
   be treated with respect. The Talmud (Shevuot 35a-b) makes it clear
   that this prohibition applies only to seven Biblical names of G-d and
   not to other names or attributes of G-d, which may be freely written.
   The prohibition was later codified by Maimonides (Mishneh Torah,
   Yesodei HaTorah 6:1-2). The practice of writing "G-d" is supported in
   Shut Achiezer, 3:32, end, where it is endorsed and accepted as the
   prevailing custom. Rambam cites Deut. 12-03:04, which states "and you
   shall destroy the names of pagan gods from their places. You shall not
   do similarly to G-d your Lord." The intent of this is to create an
   atmosphere of respect for G-d's name vs pagan gods names.
   
   As a result of this, people acquired the habit of not writing the full
   name down in the first place. Strictly speaking, this only applies to
   Hebrew on a permanent medium, but many people are careful beyond the
   minimum, and have applied it to non-Hebrew languages. Hence, "G-d".
   One explanation is that using G-d is a reminder that anything which we
   may say about G-d is necessarily metaphorical. Spelling out the Name
   (even in a language other than Hebrew) would imply that one could
   speak meaningfully (not just metaphorically) about G-d.
   
   However, the Shach (Yoreh De'a 179:11) ruled that "God" spelled in a
   foreign language does NOT have the status of a "shem" and thus may be
   erased, lehatkhila. There is a story about Rav Soloveitchik (z"l)
   intentionally writing GOD on the board while teaching a class and then
   just as deliberately and intentionally erasing it, so as to
   demonstrate by his own example that this was not a halakhically a
   problem.
   
   Conservative (ref:
   [5]http://communities.msn.com/JudaismFAQs&naventryid=160) and Reform
   practice is to use "God". However, even some who are not strict (or
   even observant) in general will write "G-d", to emphasize that Jewish
   conceptions of G-d are meant.
   
   Note: There is one exception to the destruction of G-d's name. In
   Numbers 6, the Suspected Wife Ceremony, a man who suspects his wife of
   adultery (with witnesses seeing a forbidden seclusion) brings his wife
   to the temple. The Priests test the women by pronouncing the horrible
   Biblical curse. After reading the curse it is written on parchment and
   dissolved in water (which the women drinks). If she is guilty she dies
   and otherwise the couple gets their marriage back. Thus, G-d actually
   allows the ineffable name to be dissolved in water that the women
   drinks. As the Talmud notes: G-d allows the ineffable name to be
   erased for the sake of bringing peace between a husband and wife.
   
   Note that if you disagree with another poster's decision to omit or
   include the hyphen, you should not publicly criticize or ridicule said
   poster.