Well, “何ですか” (or in romaji “nan desu ka”) means “what is it”. You can use it just like that, or add it to a sentece, for example “これは何ですか” (“kore wa nan desu ka”) = what is this.
なの is kind of a conclusion used at the end of explanation with a calm/quit sense. Example., 彼女は大学生なのよ She is university student, you know. It is just combination of two particles な and の. なのです is polite form, and なの is same with なのだ just omitting だ after that.