1. The conjugate acid-base pairs are HCl/Cl- and NH3/NH4+. They can be changed into one another by proton transfer.
2. The acid in the reaction is the molecule that has the lowest pKa value or (simply) gives their proton away the easiest. The pKa of HCl is around 3 I think while ammonia’s is around 35. This means HCl is the acid here. In the other reaction (backwards) you can see that NH4+ gives a proton to the chloride ion. To give a proton = acid. So the acids are HCl and NH4+ and the bases are NH3 and Cl-.
An important rule is: The stronger the acid/base, the weaker its conjugate base/acid.
