In order for a merchant to be Taler-compatible, they need to run two distinct Web services: a frontend and a backend. The former is typically the Web site where the merchant exposes their goods, whereas the latter is a C program in charge of making all the Taler-related cryptography.
In details, the frontend gathers all the information from customers about sales, and forwards it to the backend via its RESTful API. Typically, the backend will either cryptographically process this data or just forward it to the exchange.
That saves the frontend developers from dealing with cryptography in scripting languages and from commmunicating at all with any exchange.
Additionally, the backend RESTful API is such that a frontend might be run completely database-less.