The Communion of Saints is a very important doctrine to Christianity that states that all who are saved in Christ are part of a mystical connection, that they are apart of the body of Christ, with Jesus being the head.
The Communion of Saints essentially states that all those who are saved through Jesus Christ, whether alive in Heaven or alive on earth, are part of the mystical body of Christ. Anybody who is part of this has the Holy Ghost living within them, and are part of the Holy Church founded by and governed by Christ Jesus. Thus, the universal church and the Communion of Saints are one and the same.
Angels are sometimes thought to be a part of the Communion of Saints, because they are holy creatures and were created under the power of Christ. They do not sin, and live within Heaven, so it is accepted by many that they are members of the church.