Ammonium is the conjugate acid of ammonia. In the lab it is encountered in many salts, some of the most common are ammonium chloride and ammonium sulfate. Ammonium hydroxide is a solution of ammonia in water. Solutions of ammonium salts can release free ammonia when mixed with strong bases. Ammonium hydroxide is almost constantly releasing free ammonia to the atmosphere.
Ammonia is flammable, but difficult to ignite under most circumstances, however it can react with finely divided platinum. The major hazard associated with ammonia is its noxious choking vapors which can severely irritate the eyes and respiratory system. Never use concentrated ammonia solutions outside of a functioning fume hood.