The image above is HDR but probably not taken with a polariser. So what is HDR and how to achieve it? Well in short, ish see below.
If your photographing a static image, ie a landscape, use a tripod and take a series of exposures of the scene to capture the highlights, mid-tones and shadows correctly. With normal photography you just capture the average scene luminance in the mid-toned range and allow the highlights to burn out and the shadows to black out.
With a subject that moves ie the sea you take one image but in a RAW format. This allows you to re expose the image in post production (on the computer) recording the highlights, mid-tones and shadows as before but from one image.
Once you have all the images recorded you merge them together in Photoshop etc to achieve the desired look. This then shows more detail in the highlights and shadows to give a greater range in the photograph overall.