The language learnt comes out of the linguistic demands of the activity. A task-based syllabus is structured around a series of these tasks.
Example
A teacher uses a series of projects on British culture as a syllabus for teenage learners on a summer course in the UK, and applies the task-based approach to the work the learners do.
In the classroom
Various elements of the task-based approach are applicable to activities in other methodologies. For example, learners can see a model of the activity they are to do first, prepare a report of how they completed a task, or a project, and the teacher can record this report and analyse it for further work.
See also:
https://www.teachingenglish.org.uk/article/grammatical-syllabus
Further links:
https://www.teachingenglish.org.uk/article/a-task-based-approach
https://www.teachingenglish.org.uk/article/unit-9-task-based-learning
https://www.teachingenglish.org.uk/article/task-based-speaking
https://www.teachingenglish.org.uk/blogs/jane-willis/grammar-based-teaching-task-based-teaching-making-shift
https://www.teachingenglish.org.uk/article/criteria-identifying-tasks-tbl