In　the　dynamic　landscape　of　artificial　photosynthesis　research,　the　recent　focus　has　both　on　the　inorganic　semiconductors　and　the　polymeric　materials.　Carbon　nitrides　(CNs),　with　melon　as　a　noteworthy　representative,　have　emerged　as　promising　candidates　for　polymeric　semiconductors　and　photocatalysts.　However,　the　reported　CNs　are　often　misidentified　and　mispresented.　This　review　emphasizes　the　classification　of　the　CN　family　from　the　formation　process　and　the　characterization　of　these　materials,　shedding　light　on　the　distinctions　between　ionothermal　and　conventional　synthesis　via　thermal　condensation.　Five　types　of　CNs　are　first　distinguished　based　on　their　2D　molecular　structures.　Then,　the　corresponding　synthesis　process,　characterization,　and　photocatalytic　applications　are　summarized.　Furthermore,　the　role　of　different　ions　in　crystalline　PTI-CN　and　PHI-CN　structures　is　discussed.　We　conclude　with　a　discussion　on　identifying　the　reported　CNs　by　combining　design　and　the　synthesis　process.　©　2024　Elsevier　B.V.