Both of these phrases mean "it is" or "this is," so what is the difference between them? When should one be used over the other?
Il est can either be designating someone (therefore meaning “he is”, rather than “it is”, as is c'est), or on the contrary be a construct in which there is no subject (as in Il est tard, or Il est possible que …, etc.).
In the latter cases, you can distinguish the cases when c'est refers to a neutral subject (“it is”, really ; e.g. c'est tard will mean “this event is late”) from the cases when there really is no subject (“it rains”).
I can't think of any case where “this is” would be the translation of “il est” ; maybe your question is all about “ça / il” being the translation of “it / he”.
When this applies to a person (masculine), 'il est' is the only option.
When this applies to a concept / idea where there can be a confusion with a person, 'c'est ' is the correct choice: 'c'est malheureux' and 'il est malheureux' do have different meanings, and can lead to confusion, when one is talking\writing about a person.
Out of this ambiguitous situation, 'c'est' and 'il est' belong to different levels of language: 'c'est' is perceived as more popular, and does not require further information. 'il est' is perceived as more sophisticated, and might need some further information:
- 'c'est malheureux' \ 'il est malheureux' : 1st option applies to an idea, a situation, 2nd to a person.
- 'c'est tard' \ 'il est tard' are strictly equivalent in terms of meaning
- 'c'est malheureux' \ 'il est malheureux que ...' apply both to an idea, but the 2nd option will require additional information. It is still possible to say\write 'c'est malheureux que ...' but this would definitely be considered as a poor wording.