Pull Day
Pull day targets the opposite of push day. It mainly focuses on the back and biceps, however many back lifts also indirectly target the shoulders, mainly the rear deltoids. Many people neglect back because they don't think it's important. Hitting back will fill out your frame greatly, as well as provide extra strength and stability on lifts like bench press (if that's your goal).
Back
When hitting pull day, back is general the main muscle group lifters focus on. This is because the back is made up of many muscles, and almost all back lifts also hit the biceps. Some common back lifts include pull-ups, variations of rows, and lat pulldowns. When hitting back, it is important to make sure you hit each muscle group and don't neglect any. This can be achieved by performing a variety of lifts at different angles and grips to activate different muscle groups.
Biceps
Biceps are what many beginners are eager to train because of the aesthetics. As mentioned above, many back lifts hit the biceps, but it is still beneficial to do some bicep focused lifts. Some of the most popular bicep lifts are focused curls, preacher curls, and hammer curls. It may feel like you're doing the same thing over and over by performing many curls, but the angle and variations greatly affect which heads of the bicep are being hit. Just like all muscle groups, it is important to switch up the exercises and focus on each individual muscle equally to build the most muscle.