(Some?) Jews believe it is improper to utter the name of God, or even to write the name of God. It's even extended to whatever name is being used to identify God, even if it's not God's actual name.
So, to avoid offending the deity, vowels are removed from any spelling of a name that is used to refer to God. In the case of "God", this means spelling it "G-d". In the case of "Lord", it's "L-rd".