Normal are one of several ways to create the illusion of detail in an otherwise low-poly model.

Wiki provides a good introduction.

http://en.wikipedia.org/wiki/Normal_mapping

At the bottom are links to the other important/related detail-illusion techniques: texture, bump, parallax, and displacement mapping.

As to which is best, there is no answer. Depends on how much importance you place on different factors.

Displacement is the only one that actually changes the geometry, causing real protrusions that can be seen in profile and produce shadows falling not just on the object itself but other objects. It is also the slowest and most memory hungry technique. So is that best or worst? No single answer suffices.

Normal mapping is the only one that requires almost no computing resources, very low memory resources, works pretty well even with low-resolution maps, and requires no user-adjustment for accuracy. It is also incapable of real shape change, is not straightforward to adjust in strength, is unhelpful in providing data for height-based procedural shader effects, and doesn't produce any shadow or profile variation. So is that best or worst? No single answer here either.

As to why Poser models don't come with N-Maps; it is that only the most recent versions of Poser support it. Vendors avoid making products that only work with recent versions because it cuts down on sales. It is also harder to make N-Maps. You can't just draw them like you can bump maps. Tools like Z-Brush produce them for you but most vendors don't have that in their workflow.

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