A defocused image of a bright single star in a small telescope contains rich information on the optical turbulence, i.e. the seeing. The concept of a novel turbulence monitor based on recording sequences of ring-like intrafocal images and their analysis is presented. It can be implemented using standard inexpensive telescopes and cameras. Statistics of intensity fluctuations in the rings and their radial motion allow measurement of the low-resolution turbulence profile, the total seeing, and the atmospheric time-constant. The algorithm of processing the images and extracting the turbulence parameters is developed and extensively tested by numerical simulation. Prescriptions to correct for finite exposure time and partially saturated scintillation are given. A prototype instrument with a 0.13-m aperture was tested on the sky. The RINGSS (Ring-Image Next Generation Turbulence Sensor) can be used as a portable turbulence monitor for site testing and as an upgrade of existing seeing monitors.