Like all academic writing, an essay should be based on research, with sources integrated and clearly documented through referencing. As with other modes of academic writing, an essay should demonstrate critical thinking.
Unlike the other genres of academic writing you may produce for GSA, an important feature of an essay is that it puts forward a main argument (or thesis). A good essay will also show consideration of alternative arguments and perspectives, while demonstrating that your argument is the preferable one.
See also this British Council resource on the essay genre.
Next page: essay writing processes