There is no difference in the effect on the final image. The difference is in the technical way that the filter manages the light at wavelength level. Linear polarisers provide standard polarised light with all lightwaves vibrating in the same plane, whereas circular polarisers will polarise and then spin the lightwaves. The internal optics that control focussing or metering in some modern cameras cannot work with standard polarised light and therefore a circular polariser is required. The information on which type is needed can normally be found in the instruction book for your camera.