All our knives are rated for how hard they are with what is called the Rockwell rating or measurement (aka HRC) of the steel.
When discussing how hard something is it can be difficult to be precise about it unless you have a measurement. I can say as soft as butter for instance and you'd have a good idea but that softness depends on if it's a hot day or a hot kitchen, if it's fresh out the fridge etc. They all change something we think of as soft into something either gooey or impossible to spread on bread! What we need is a measurement, a number, a scale to let us know how hard something is. Step in the Rockwell hardness scale and the one we are interested in for kitchen knives is the one call "HRC".
HRC is clever in its simplicity. The short easy version is it measures how much of a dent/mark a diamond point can make in the metal with a measured amount of weight. The smaller the mark the harder the steel. If you want to get all geeky about it read more about the Rockwell Hardness Scale on wikipedia.
For good kitchen knives (ie the ones we stock) you will likely see ratings from 60-66HRC. For a comparison an axe would be about 50HRC, your super market special knives are 54-58HRC.
The higher the rating the harder the material so the thinner/finer the cutting edge can be. Which is why you see higher ratings on our knives as they are built to be sharper.
So you might be thinking "higher is better" and as a rule of thumb it is but as ever there are trade offs. The harder the steel the more fragile it can be so high steel knives are more likely to chip if misused. This can be countered somewhat by the choice of steel; powder steels can have a very high HRC rating (64-66HRC is common such as our Kotetsu range). Also the harder a steel the longer they can take to sharpen but they stay sharper for longer (which is one of the whole points of getting a good knife for life, more time spent cooking rather than sharpening).