CCC is the Catechism of the Catholic Church, and the number is the section (paragraph) number. CIC is the code of canon law for the Latin Rite (the abbreviation of from the Latin title of the Code: Codex Iuris Canonici), and the number is the canon number. CCEO is the code of canon law for the Eastern Rites of the Church (abbreviating the title Corpus Canonum Ecclesiarum Orientalium) , and again the number is the canon in question. DS is a volume of excerpts from historic Church documents and is known as “Denzinger-Schönmetzer” (after its editors), but the actual title is Enchiridion Symbolorum, and the number in such citations is the excerpt number within the book. These volumes constitute four of the most important resources for Catholic doctrine, morals, and law, and so they get quoted (and abbreviated) often.