Microsoft Small Business Server is actually software for several servers but with limitations.
It comes in two versions: Standard and Premium. It supplies all the funcionality of the indiviual servers but has two primary limitations; it limits the nunber of users (usually 50 or less); it prevents you connecting more than one SBS together or to another network.
However for the SME with less than 50 staff it is a real bargain in that it provides all the features of a multiple server environment at a fraction of the cost where you to buy licences for each server in the family.
The chart below shows the features of SBS and the differences between the two editions.

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