Kedusha means that you are making "Kodosh" the Holy Name of Hashem. This happens when the chazan repeats the Shemoneh Esrei along with a minyan who say "Kodosh, Kodosh, Kodosh" together. Everyone says Kedusha when the minyan does, and not before or after. They stand with their feet together, and pick up their heels when they say "Kodosh" and "Baruch Shem Kovod." They are standing like the angels that flap their wings when they say Kedusha. No talking is allowed in the middle of Kedusha. If someone is davening at the time and he is not yet saying Shemoneh Esrei, he should stop and say Kedusha with the minyan.