Deciding between complaint or grievance can sometimes be difficult and may often be a judgment call on the behalf of the receiver. As a guideline, a conversation with a student is usually considered an informal grievance, whilst a written letter is usually considered a formal complaint. The tricky one to define is email, as an email can take the form of either a casual conversation or a formal letter of complaint. All you can do is use judgement based on the content of the email.
If you have receive an informal grievance from a student and after casual conversation it remains unresolved, you must inform the student of their right to lodge a formal complaint, either by writing a letter to the appropriate person based on the nature of the complaint (you should help the student determine this person) or simply by filling in this form.