Morphological knowledge is represented in the actual characters in a word. The syntax of a language restricts the possible combination of words in a language by allowing some orders of words and disallowing others.
Semantic knowledge is the symbolic 'meaning' of words.
The semantics of a word often depends on its syntax and context. Discourse knowledge is a higher level of abstraction for semantic knowledge where the information of groups of words, sentences, paragraphs, etc is represented symbolically. How lexicons should be designed for computerized analysis of text and speech is an important area of NLP research. In the medical domain, we often see words never used in normal conversation, as well as 'normal' words used in ways very different from their conventional use. Likewise, many words which occur in everyday discourse are never used in the medical domain.