Ordinary People is the story of both Conrad and Calvin Jarrett. Because the novel focuses on two different people, there are several conflicts throughout the novel that are specific to those individuals. The central question in Conrad's story is whether he will be able to recover after his suicide attempt. As Dr. Berger points out, half the people who attempt suicide will try to do it again at some point in their lives. The inclusion of Karen's suicide towards the end of the novel is a way of reminding the reader that Conrad may not have recovered completely even when he seems to be getting better; after all, Karen seemed to be doing well when Conrad met her for a Coke earlier in the novel.
The main question in Calvin's story is whether he and Beth will be able to make amends. Their conflict is based essentially in a communication problem: Calvin believes that the way to heal the wounds of the past is to talk through them and discuss feelings, while Beth only wants to move on from the past. She dislikes Calvin's attitude and his insistence on worrying about his son. The conflict between the two parents is resolved at the end of the novel when Beth leaves.
Structurally, the novel does two things. First, it alternates back and forth between the stories of Calvin and Conrad, with each chapter shedding some new light on their individual struggles and conflicts. This alternating style gives the novel a kind of mirror-image structure: just as Conrad gets better over the course of the novel until he is really healed, the marriage between Calvin and Beth spirals downward until it fails.
The second structural tactic of the novel is that it begins in a world that is already in some way ruined: Buck has already died, and Conrad has already tried to commit suicide even before the first chapter opens. On the one hand, this indicates that the book is a novel about healing and rebuilding a ruined world, rather than about how that world got ruined in the first place. This structure, however, also gives the book a reverse coming-of-age feel. There are countless children's books about boys who begin the novel as innocent kids and after a series of life experiences end the novel as slightly more mature and wiser young adults (Huckleberry Finn and The Catcher in the Rye are examples.) Ordinary People tells a coming-of-age story backwards. Conrad has already been through his moment of great experience--the death of Buck--and the novel is really the story of how he tries to move on from that horrible moment back to a state of some youthful innocence once again. Ordinary People is in this sense a subversion of one of the most oft-used forms of narrative in English literature.
Indeed, the alternating chapters include many flashbacks to moments from the past. These flashbacks show that Guest is very much interested in the "moment of experience." Calvin and Conrad retain certain key memories of specific moments in their lives, most of which are relatively unimportant. Particularly in Calvin's introspective chapters, we see some of these memories emerge. Ordinary People illustrates the idea that humans always undergo moments of experience, many of which we do not even understand until we look back on them from the future. Many of the moments portrayed in the novel seem to show that the present is a blur that we do not really understand until it has become the past. Memories play a major part in the characterizations in Ordinary People.
There are several main themes in Ordinary People, one of which is that healing after even the most horrific experiences is possible on an individual level, but difficult in a group context. Conrad himself manages to get better; Calvin and Beth's marriage does not. The reason for the difficulty is communication. Beth and Calvin simply lose their ability to communicate effectively with one another, because they believe that communication ought to occur very differently. Nevertheless, another theme of Ordinary People seems to be that in order to heal, real communication needs to take place. Conrad really gets better only after his outburst with Dr. Berger about everything that has been troubling him. Calvin and Beth never truly communicate, and so their marriage never heals.
Finally, Ordinary People can be seen as the story of how two people form a meaningful relationship with one another. The relationship between Conrad and Calvin begins in shambles. Conrad does not appreciate his father, and Calvin never ceases to indulge his son. At the end of the novel, after Conrad has healed and Calvin has split with Beth, they come together in the final chapter, understanding one another for the first time. For all its depressing moments and disturbing psychological implications, Ordinary People is an optimistic novel: it shows that a meaningful father-son relationship can emerge despite a terrible past and problems with communication. If the novel begins in a ruined world, it also shows that ruin need not be permanent.