"He is a bad boy."
Translation:C'est un mauvais garçon.
The best differentiation I have found is here: http://www.frenchtoday.com/blog/cest-versus-il-elle-est
It requires close reading several times!
Briefly, "C'est" precedes a noun, so "C'est un mauvais garçon,"
"Il est" precedes adjectives, so "Il est mauvais."
As someone noted below, formal French might use "il est" where "c'est" is used, but apparently it sounds wrong nowadays in spoken French.
Read the full page at the above link; it's worth the time.
In this case, more context is require to differentiate those two sentences. "Ceci est un mauvais garçon ." - This is a bad boy "C'est un mauvais garçon ." He/This/That is a bad boy. You could say it either way, but the former sentence is if you want to emphasize/specify the difference.<pre>
Disregard the fact that you posted this question two years ago and obviously know the difference by now but for the people who are starting, this should help you.</pre>
Rules regarding the verb to be.
Nouns: Unmodified nouns invoke the il/elle est form. Modified nouns invoke the c'est form. Most nouns require a modifier in French. Professions don't require a modifier in French.
He is a professor = allows an unmodified noun structure = Il est professeur
He is a boy = requires an article thus the modified noun structure = C'est un garçon.
Adjectives and Adverbs. Stand alone adjectives and adverbs can refer to a person or a situation. Those that describe a person require the il est form. Those describing a situation invoke c'est.
He is bad = describes person = il est mauvais.
It is bad = describes situation = c'est mauvais
See http://french.about.com/library/weekly/aa032500.htm for a better explanation.
"Mauvais" is one of those adjectives that goes before the noun—that's just the way it is. It can't be used after "garcon." Most adjective go after the noun. Some can go in both positions and the meaning changes.
See here for a list of adjectives that go before the noun: http://www.bbc.co.uk/bitesize/standard/french/grammar/adjectives/revision/1/
"C'est" refers to any noun on the predicate side of a sentence: "This/that/it/she/he is..." C'est Père Noël, c'est un lapin, c'est une fille." etc.
That's because in French you don't use the form Il est + noun, only Il est + adjectif. For nouns you use C'est instead, as in this sentence. This article explains it better than I can: http://www.frenchtoday.com/blog/cest-versus-il-elle-est