It can be compared to formal assessment, which involves evaluating a learner's level of language in a formal way, such as through an exam or structured continuous assessment.
Example
The teacher has carried out informal assessment of the learners during speaking activities throughout the course by keeping notes about how well individuals are doing in speaking and uses the data gathered to evaluate the learners at the end of the course.
In the classroom
Informal and formal assessments are both useful for making valid and useful assessments of learners' knowledge and performance. Many teachers combine the two, for example by evaluating one skill using informal assessment such as observing group work, and another using formal tools, for example a discrete item grammar test.
Further links:
https://www.teachingenglish.org.uk/article/assessment