What is the difference between the names of "Jehovah God" and "The Lord"?

I noticed that the American Standard Version of the Bible describes God in a couple different ways. For example, Genesis 3:8: > And they heard the voice of Jehovah God walking in the garden in the cool of the day: and the man and his wife hid themselves from the presence of Jehovah God amongst the trees of the garden. compared to Genesis 18:30 > And he said, Oh let not the Lord be angry, and I will speak: peradventure there shall thirty be found there. And he said, I will not do it, if I find thirty there. In the Old Testament, what is the difference between the names of "Jehovah God" and "the Lord"?