What is a quotient structure? I was reading a paper here, and it mentioned "quotient structure" in the following sentence (third page, second paragraph of the paper)


  In order to obtain a representation of terms truly isomorphic to
  λ-terms, we need to build a quotient structure, quotient the set of
  raw terms with respect to alpha-equivalence. This construction based
  on a quotient corresponds very closely to the of presentation from
  standard textbooks on λ-calculus.


I am hoping that someone could point me a good tutorial on quotient structures.
It's a term coming from abstract algebra. When you quotient a set $S$ with respect to some equivalence relation $\sim$, you are replacing the set $S$ with its equivalence classes under $\sim$. Often the set $S$ has operations defined on it, and in that case we want these operations to be well defined for the quotient $S/\sim$, in the sense that if $x_i \sim y_i$ then $x_1 \circ x_2 \sim y_1 \circ y_2$. This allows us to define $\circ$ on the quotient.

An example in real analysis is $L^p$ spaces, in which functions are defined "up to measure zero".