What the Numbers Mean
The acronym ISO stands for International Organisation for Standardisation, yes O and S are the wrong way round, it is just conferring a level of legally accepted standardisation. I believe this standard identification was adopted in 1974 combining ASA and DIN ratings for film sensitivity, although these remained in existence for several years after the adoption. The era of digital photography adopted the Standard and all that the numbers associated with ISO actually mean is the level of camera sensor sensitivity. Most digital cameras have a base ISO of 100 with the highest being 12,800 or 25,600. Photographers used to talk about a slow rated film (ISO 25) or fast which was ISO 800, the faster the film the more grain - film is round dots, digital is pixels - squares; digitally we now talk about noise, where the higher the ISO number the more noise.
The key to all this is to try and use the lowest ISO number possible given the lighting conditions, thereby making sure there is minimal noise in the resultant image. Changing the Aperture or Shutter value offer the means to achieve this around a given ISO, but of course subject matter and the objective behind the required image are also key factors. To unravel all this, the full stops are: ISO 100, 200, 400, 800, 1600, 3200, 6400, 12,800 and 25,800, but most cameras provide third stop settings, for example from ISO 800, then next is 1000, then 1250 and 1600. This allows some quite fine camera adjustment to achieving an evenly exposed image in most light conditions. The cost of pushing the ISO higher and higher is noise, demonstrated by these six images. Starting at ISO 200 the noise in the lettering and cymbal lathing becomes more obvious as in the final image where the ISO was 25600. This may be a desired effect and aim although it can be distracting if an image is seeking to outline very fine detail of a subject. (Hover Cursor For ISO number).