Paradox (in return for a royalty) markets SSE, they also provide distrobution channels (steam and on DVD).
Think of it as Vstep being a company you've bought something from, and Paradox being the courier
Unless vstep has entered into a very unusual arrangement, this is somewhat wide of the mark. The relationship between creator - publisher - customer can take a number of different systems (for example publisher can buy X from creator and then re sell it to the customer -OR- the publisher can act as an agent for the creator, which is similar (similar, not the same) as described above by Danny.)
It is quite important because there is a difference in law between the two- it defines who your contract is with* paradox or vstep.
* Actually, that isn't true as your contract is with the retailer... If you have a complaint with the product your contract is actually with the retailer- thus in principle THEY are the ones to whom you should theoretically report bugs to and it is for THEM to resolve it with paravstepdox. Theoretically at least.