There are a few different ways of saying "when" in French, the most basic of which is quand. Like "when," quand can either be an adverb or a conjunction. As an adverb, it's generally used to form questions:

Quand seras-tu libre?
When will you be free?

Tu l'as inventé quand ce morceau?

When did you compose this piece?

Caption 24, Claire et Philippe - Mon morceau de piano

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BANNER PLACEHOLDER

À quelle heure is an adverbial expression that's more or less synonymous with quand, albeit a bit more specific. It's the equivalent of "at what time" in English:

Enfin, tu commences à quelle heure le travail?

Anyway, what time (when) do you start work?

Caption 70, Elisa et Mashal - Petit-déjeuner

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As a conjunction, quand is synonymous with lorsque:

À Paris quand vous sortez le soir,

In Paris when you go out at night,

le métro se termine à minuit trente.

the metro stops [running] at half past midnight.

Captions 15-16, Amal - Vélib

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Lorsque je vous vois, je tressaille

When I see you, I quiver

Caption 19, Bertrand Pierre - Si vous n'avez rien à me dire

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We could easily switch quand and lorsque in those examples:

À Paris lorsque vous sortez le soir, le métro se termine à minuit trente.
Quand je vous vois, je tressaille

However, you can't use lorsque as an adverb, that is, as a question word. So you would never ask someone, Lorsque seras-tu libre?

You'll also see the phrase au moment où ("at the moment when") instead of quand or lorsque:

Au moment où le chat sortit en courant,

When the cat ran out,

la calèche royale atteignait le château.

the royal carriage reached the castle.

Captions 33-34, Contes de fées - Le chat botté

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Où usually means "where," but sometimes, as in au moment où, it means "when":

Les lignes de métro vont s'ouvrir

The subway lines will open [continued to open]

jusqu'à mille neuf cent quatre-vingt-dix,

until nineteen ninety,

dans les années mille neuf cent quatre-vingt-dix,

in the nineteen nineties,

où la ligne quatorze fut ouverte.

when line fourteen was opened.

Captions 17-20, Adrien - Le métro parisien

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Le dimanche, où les gens ne travaillent pas,

Sunday, when people don't work,

on va prendre le croissant, on va prendre le pain au chocolat.

we'll have a croissant, we'll have a chocolate croissant.

Captions 29-30, Arles - Le petit déjeuner

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BANNER PLACEHOLDER

If you're ever in doubt when to use which word for "when," just go with quand. It has the broadest scope, so you can use it pretty much n'importe quand (whenever).