When you refer to Spirit Guide, you refer to the belief that there is a separate entity from the person, contactable through dreams or other methods, that helps to guide them. You can read more about this and ask questions in the the BIG Spirit Guide topic III topic.
When you say DG or dream guide you refer to the idea of a character in your dream that helps to guide you, this doesn’t necessarily assume a spiritual belief as it could be part of your sub-conscious.