Writing "G-d" instead of God is a fairly recent custom in America. Many believe this to be a sign of respect, and the custom comes from an interpretation of the commandment in Deuteronomy 12:3-4 regarding the destruction of pagan altars. According to the the medieval commentator, Rashi, we should not erase or destroy God’s name and should avoid writing it. In a Responsa (legal opinion) by the Central Conference of American Rabbis, the primary prohibition against erasure of the name God applies to the sacred names in a written text of Torah. With the advent of computers and the internet, rabbinic authorities have debated whether or not this applies to what is typed on a computer or read on a screen. Most have concurred that it does not apply.
The bulk of Jewish legal opinion agrees that the law applies only to the written name of God when written in Hebrew and not when written in other languages. Reform Jewish practice reflects this opinion. Some Jews will avoid discarding paper or books in which God’s name appears in Hebrew. Rather than being thrown out or destroyed, they may be stored in a genizah (a storage place) and buried in a Jewish cemetery.