There are 2 types of licences available for premium themes. Personal licences are for private individuals for use on up to 4 blogs, commercial licences are for businesses and profit-making entities and are for use on up to 20 blogs. Additionally, commercially-licensed themes don’t include a theme credit icon (present in the top right corner on free and personally-licensed themes).
We have a tracking system which stops theme purchases being abused and mass distribution, if your theme code is recorded as being used on more blogs than it’s licenced for we have a right to remotely remove its advanced features.