Reflex is an fully-deterministic, higher-order Functional Reactive Programming (FRP) interface and an engine that efficiently implements that interface.
Reflex-DOM is a framework built on Reflex that facilitates the development of web pages, including highly-interactive single-page apps.
Comprehensive documentation is still a work in progress, but a tutorial for Reflex and Reflex-DOM is available at https://github.com/reflex-frp/reflex-platform and an introductory talk given at the New York Haskell Meetup is available here: Part 1 / Part 2.
A summary of Reflex functions is available in the quick reference.