*¿Cuál es la diferencia entre tú y usted?
If in the question you use a conjugated form of 'ser', 'cuál(es)' is the question word to use.
'Tú´(with accent, as 'tu' means something else) is the personal pronoun for the second person singular i.e. 'you', used in 'informal' contexts. 'Usted' by crux, is the formal way to refer to someone as 'you' in the singular. Elderly people, your superior at work, customers in the service industry, all could be referred to as 'usted' instead of 'tú'. Now, the choice you use directly affects the verb conjugation.In your question you have used 'corrija' which is the usted form of the imperative. '(Tú) corrige' would be the informal way.
Now, the usage of these words is country dependent. In Spain 'usted' is not used very much. For someone who you don't know of the same age, I haven't heard 'usted' being used in Spain, for example. And in some countries, 'usted' (e.g. in parts of Colombia) is used even in informal contexts. There is also a third way to refer to someone informally, 'vos' which is typically used in the Rioplatense region in Argentina and Uruguay.