I have edited your question to fit enotes requirements. Remember, you are only allowed to ask one question per day. As with any work of literature, there can be any number of identifiable themes. The Giver is no different in inviting a wide number of different interpretations that might focus on topics such as what it is to be human, the relationship between pain and pleasure, and the role of the individual. For me, however, more important than these themes is the role of memory.
What is interesting about this book is that the author was inspired to write it after visiting her aging father who had lost his long term memory. This led her to realise that without memory there is an absence of pain. If you forget experiences that caused pain in your past life it is as if the pain did not exist. Jonas is part of a community that at some stage decided to eradicate pain by eradicating memory. This allows them to create the perfect utopian society, where members are free to live in happiness, but also are free of urges to engage in any activities that will bring unhappiness to them and the community of which they are a part. As Jonas discovers, however, memory (and the suffering that often accompanies it) is necessary to truly live - the half lives that his friends and family live as depicted in the novel make this truly clear. The unescapable message of this novel is that pain and suffering is part of the human condition and makes life the rich tapestry of experiences that it is. If we forsake our ability to suffer by eradicating memory, we are not truly human.