Lternative type of assessment : Observations

I need to write a research paper about one type of alternative assessment for ESL learners which is (Observations ).
what do we mean by it ?who is using it ? how can we apply it in the ESL class? what things should be observed ?and any other related questions .

Note 1: please use at least 5 sources .
Note 2 : please use a very simple language since I am not an English native speaker.