C'est and il/elle est are two common expressions used to describe people or things in French. Though they have the same meaning (he/she/it is), they're not interchangeable. So how do you know when to use which? It all depends on what comes after the verb est (is). Let's look at some examples.
Il est (masculine) and elle est (feminine) are primarily used before an adjective alone, or before an adverb and adjective (such as très intelligent):
Il s'appelle André. Il est très intelligent.
His name is André. He's very smart.
They're also used to describe someone's nationality, religion, or profession:
Elle est japonaise. Elle est bouddhiste. Elle est chimiste.
She is Japanese. She is Buddhist. She is a chemist.
Note the difference between the French and the English in that last sentence. You don't need an indefinite article (un, une) after il/elle est when talking about someone's profession. So you don't say elle est une chimiste, but simply elle est chimiste.
C'est is used in pretty much every other circumstance. You'll find it before a modified noun, such as mon ami:
Il s'appelle André. C'est mon ami. [Not: il est mon ami.]
His name is André. He's my friend.
Or before a disjunctive pronoun (moi, toi, lui, etc.):
Ah, oui, c'est moi. -C'est toi mais c'est vrai!
Oh, yes, it's me. -It's you, but it's true!Play Caption
L'État, c'est moi.
The State, it is I (or "I am the State").
(attributed to King Louis XIV of France)
C'est can also come before a standalone adjective (such as c'est vrai in the example above), but only when you're making a general statement about a situation. If you're referring to something specific, then you use il/elle est:
Cette histoire n'est pas inventée. Elle est vraie.
This story isn't made-up. It's true.
If you're describing a group of people or things, then you need to use the plural forms of c'est and il/elle est. These are ce sont and ils/elles sont (they are):
Ah, ce sont les fameuses pommes de terre, euh... violettes.
Oh, these are the famous, uh... purple potatoes.Play Caption
Ne vous approchez pas des ours. Ils sont très dangereux.
Don't go near the bears. They are very dangerous.
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