An informal essay is written mainly for enjoyment. This is not to say that it cannot be informative or persuasive; however, it is less a formal statement than a relaxed expression of opinion, observation, humour or pleasure. A good informal essay has a relaxed style but retains a strong structure, though that structure may be less rigid than in a formal paper.
The informal essay tends to be more personal than the formal, even though both may express subjective opinions. In a formal essay the writer is a silent presence behind the words, while in an informal essay the writer is speaking directly to the reader in a conversational style. If you are writing informally, try to maintain a sense of your own personality. Do not worry about sounding academic, but avoid sloppiness.