The Canonical Text Services protocol defines interaction between a client and server providing identification of texts and retrieval of canonically cited passages of texts. The official specifications by David Neel Smith and Christopher Blackwell can be found here.
To put it relatively simple: CTS serves text passages that are specified by URN like references. It is specified in a way that allows to create CTS URNs for any possible text passage in a document.
The data can be requested using GET requests that are provided in an URL. Each request must contain one parameter request which specifies the CTS function to use. Function specific parameters - like the URN - are added as additional GET parameters.
For example, the following CTS request returns the text content of chapter 3 of the book Genesis of the English King James Bible.