talk vs discourse

talk and discourse both are nouns.

Word	Noun	Adjective	Verb	Adverb
talk	Yes	No	Yes	No
discourse	Yes	No	Yes	No

As nouns, discourse is a hypernym of talk; that is, discourse is a word with a broader meaning than talk:

talk: discussion; (`talk about' is a less formal alternative for `discussion of')
discourse: an extended communication (often interactive) dealing with some particular topic

Other hypernyms of talk include discussion, treatment.

As verbs, discourse is a hyponym of talk; that is, discourse is a word with a more specific, narrower meaning than talk:

talk (noun)	discourse (noun)
the act of giving a talk to an audience	an extended communication (often interactive) dealing with some particular topic
an exchange of ideas via conversation	extended verbal expression in speech or writing
idle gossip or rumor	an address of a religious nature (usually delivered during a church service)
a speech that is open to the public
discussion; (`talk about' is a less formal alternative for `discussion of')

talk (verb)	discourse (verb)
express in speech	talk at length and formally about a topic
exchange thoughts; talk with	carry on a conversation
reveal information	to consider or examine in speech or writing
use language
deliver a lecture or talk
divulge confidential information or secrets