This is an acid-base reaction (neutralization): NH4Cl is an acid, KOH is a base.
NH4Cl – Ammonium chloride
Other names: Sal ammoniac, Salmiac, Nushadir salt
Appearance: White solid, hygroscopic ; Odourless colourless-to-white hygroscopic solid in various forms ; Finely divided, odorless, white particulate dispersed in air.
KOH – Potassium hydroxide
Other names: Caustic potash, Lye, Potash lye
Appearance: White solid, deliquescent ; White hygroscopic solid in various forms ; Odorless, white or slightly yellow lumps, rods, flakes, sticks, or pellets. [Note: May be used as an aqueous solution.]
Names: Potassium chloride, Sylvite, Muriate of potash
Appearance: White crystalline solid ; Colourless hygroscopic crystals
Names: Ammonia, Azane (only intended for use in naming derivatives of ammonia), Hydrogen nitride
Appearance: Colourless gas ; Colourless gas or compressed liquefied gas with pungent odour ; Colorless gas with a pungent, suffocating odor. [Note: Shipped as a liquefied compressed gas. Easily liquefied under pressure.]
H2O – Water, oxidane
Other names: Water (H2O), Hydrogen hydroxide (HH or HOH), Hydrogen oxide
Appearance: White crystalline solid, almost colorless liquid with a hint of blue, colorless gas