Japanese terminology for classifying samurai swords is quite precise.
Since only samurai were allowed to carry a long sword in ancient Japan, the most important distinction is between blades that fall into this category and those that do not. The size, which is called nagasa, is calculated by going in a straight line from the point to the mune-machi, the step that separates the cutting part from the nakago, the tang that is inserted into the habaki and the hilt. The distinguishing feature is the measurement of 2 shaku, about 60 cm: beyond this measurement one speaks of "katana", while below it one speaks of "wakizashi". Swords below this threshold could be carried by everybody, so it is not uncommon to find swords that are a few millimeters below 60 cm and are instead kept in longer scabbards.
The distinction between a wakizashi and a tanto, the Japanese dagger, is a bit more fluid. Usually one shaku - 30 cm - is considered the transitional measurement, but often it is more the geometry of the blade and the way it is mounted that determines the type. In fact, it is common for blades over 30 cm to be constructed and mounted as tanto, in which case they are referred to as sunnobi-tanto, meaning a tanto that exceed the standard size of about a sun, or 3 cm.
A final basic distinction, back to longs words, is that between katana and tachi. Again, the distinction is not entirely clear, as katana also means "sword" in general. In any case, the term tachi is generally used for the oldest, longer, and more curved swords, which were originally carried on horseback with the edge down. The term katana, on the other hand, is usually used for swords worn into the obi. The fact that almost all tachi have been shortened and made into katana plays into the rather fluid terminology.