I want to know the difference between the two Stack Exchange sites 'English Language & Usage' and 'English Language Learners' (this site).
The difference between the sites is explained in detail here, but basically it boils down to the following: this site is meant
for speakers of other languages learning English
(from the Tour)
so it focuses on basic questions about grammar, word/phrase meanings, etc., which would be 'obvious' to native speakers (though this does certainly not mean they all would be able to answer them; English can be very confusing at times for no apparent reason).
On the other hand,
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts.
(from the EL&U Tour)
While we do have a lot of questions here which would be interesting/not too basic for native/fluent English speakers (e.g. this one), those would fit on EL&U as well.