Brainstorming is the random generation of ideas based around a topic. There is no editing or ordering of these ideas. They may then be used as the basis for another activity such as writing or discussion. It is often very productive as a whole-class activity.

Example
Learners brainstorm the topic of smoking by writing all the words they associate with it on the board. This is then followed by a discursive essay-writing lesson on the topic.

In the classroom
Brainstorming can encourage learners to think more freely and creatively than if they were doing a more controlled planning exercise. It allows learners to remember what they know, and to teach each other. It is a dynamic and stimulating way to lead learners into a topic.

Further links:

https://www.teachingenglish.org.uk/article/programme-5-introduction-writing

https://www.teachingenglish.org.uk/article/how-approach-discursive-writing