Semantics - the meaning of words and sentences and the varieties and changes in the meaning of language
Example: The simple word "on" can have many meanings such as: on call, on the roof, on cloud nine on edge, on fire, on purpose, on demand, on top, or on the phone

Phonology - the sound system of the language, including the sounds that are used and how they may be combined
Example: the study of the movements the body goes through in order to create sounds- such as the pronunciation of the letter "t" in "bet," where the vocal chords stop vibrating causing the "t" sound to be a result of the placement of the tongue behind the teeth and the flow of air

Morphology - units of meaning involved involved in word formation
Example: Firehouse, Doghouse, Bathroom, Chairlift... These words all separated have meanings that are different than these words put together. Like... a fire is very hot and can destroy things. A house is where you live. But a firehouse is a place where fireman work and stay in in case of an emergency. 

Pragmatics - the appropriate use  of language in different contexts
Example: Being asked "How are you today" and responding with a general statement such as "I'm fine, how are you?" instead  of responding with how you really are.