C'est and il est/elle est can both translate as "he/she/it is", but they're used in different grammatical contexts. C'est is followed by a noun (with its article) or a stressed pronoun: c'est un médecin (he's a doctor), c'est elle (it's her), c'est intéressant (it's interesting, referring to a whole situation).
Il est/elle est is followed directly by an adjective or a profession used without an article: il est médecin (he's a doctor), elle est intelligente (she's smart). The key test is what comes next — a bare noun or profession without an article calls for il/elle est, while anything with an article, a pronoun, or a general comment about a situation calls for c'est.