Learn Russian: В чём здесь разница?

When asking about the core of an abstract concept like a "difference" (разница), a "problem" (проблема), or a "meaning" (смысл), Russian uses в (in) + the Prepositional case. You are literally asking, "In what does the difference lie?"

Where is the verb "is" in this sentence?

It is omitted, which is the standard rule for the present tense of the verb "to be" in Russian. If you wanted to ask this in the past tense, you would need to add it back in: В чём здесь была разница? (What was the difference here?).

Is it possible to ask Какая здесь разница? instead?

Yes! However, Какая разница? is very often used rhetorically to mean "What difference does it make?" or "Who cares?". If you genuinely want someone to explain how things differ, В чём разница? is the best and most polite phrase to use.

This lesson is about оба and обе (both). How does this sentence fit in?

While this specific sentence doesn't contain the word for "both," it is the most natural follow-up question when comparing two items. If someone is talking to you about two choices (оба варианта) and they seem identical, this is the exact idiomatic phrase you need to ask how they differ.