Ammonium is an ion with a formula of NH4+. It is pseudo-metallic in its properties and greatly resembles alkali metals.
Free "metallic" ammonium does not exist as a pure substance, but it can be prepared as an amalgam with mercury. The amalgam of ammonium is a grey substance, often spongy (in high concentrations, ammonium spontaneously decomposes into ammonia and hydrogen, producing gas bubbles which give it its characteristic sponginess). Low-concentration amalgams of ammonium react with water, evolving hydrogen and forming a water solution of ammonia (the "ammonium hydroxide"), like alkali metals. High concentrations evolve a mixture of ammonia and hydrogen.
Both "ammonium hydroxide" (a solution of ammonia in water, in which the ionic species NH4OH is a minor tautomer) and gaseous freebase ammonia are capable of forming salts with acids. These salts look and behave like typical alkali metal salts, in that they are colorless or colored by the anion, and soluble in water. Heat causes them to decompose into ammonia and the acid, sometimes reversibly, sometimes - when the acid itself decomposes - irreversibly.
Ammonium salts of strongly oxidizing acids are known to be energetic materials.
Ammonium forms a large amount of organic derivatives, which can be divided into two groups: quaternary ammoniums and everything else. The "usual" (non-quaternary) organic ammonium derivatives should be more properly thought of as derivatives of ammonia that form ammonium-like cations: they are organic bases that can exist in freebase form, just like ammonia itself, they typically do not form stable, isolable hydroxides and are weak or mid-strength bases. Quaternary ammoniums do not exist as freebases, but form stable, isolable ionic hydroxides that behave like alkali metal hydroxides; these are typically strong bases.