Kb stands for the base dissociation constant. It measures the strength of a base in terms of its ability to accept protons (H+ ions) from water molecules and form hydroxide ions (OH-) in an aqueous solution.
pKa is the negative logarithm of the acid dissociation constant (Ka), which measures the strength of an acid. It is related to Kb because they both quantify the strength of substances in solution.
Lewis Acid: A Lewis acid is any species that can accept an electron pair. In terms of acids and bases, it refers to substances that can receive and bond with electrons.
The conjugate acid is formed when a base accepts a proton (H+) from an acid. In this case, NH4+ would be the conjugate acid formed when NH3 accepts a proton.