A D-Group is a closed group, rather than an open group. Sunday school classes and Bible studies are usually open groups (or they should be); that is, they are open to anyone and everyone who would like to attend. A D-Group is, by design, closed to a handful of people. A person joins the group by invitation only. The recommend size is 3-5 people (including the leader).
Second, the purpose of a D-Group is completely different. While Sunday school classes and Bible studies exist for the purposes of growth and fellowship, they have an underlying additional purpose (or they should have): evangelism. Sunday school classes are designed to reach lost people by getting them involved in the group. A D-Group, in contrast, consists of believers who desire a deeper walk with Christ. It is not evangelistic in its form or function, but in its fruit: it makes disciples, who then make disciples.
Lastly, the setting of the D-Group is completely different. It is the shift from a lecture atmosphere with one teacher facilitating a discussion of a room full of students to an intimate, accountable relationship with a handful of like-minded people. In their book The Invested Life, Joel Rosenberg and T. E. Koshy suggest that a discipleship relationship is “more personal, more practical, and more powerful. A teacher shares information, while a discipler shares life; a teacher aims for the head, while a discipler aims for the heart; a teacher measures knowledge, while a discipler measures faith; a teacher is an authority, while a discipler is a servant; and a teacher says, ‘Listen to me,’ while a discipler says, ‘Follow me.’” This blueprint, sketched by Jesus Christ through His personal example, is how discipleship is accomplished in the lives of believers, and, ultimately, within the local church. When this plan is followed, those involved will participate in three dynamics that result in growth in their personal lives, as well as in the Kingdom: community, accountability, and multiplication.