It depends what kind of security you want on your shares, as there's two different types of security over SMB, share-level and user-level. The difference between user-level and share-level is that share-level is purely based on passwords. Depending on your password, the system will decide what type of access to grant you. User-level security, on the other hand, grants individual users different permissions, and is thus more flexible.
Which one you choose really depends on your situation. If you've got a lot of shares and users, user-level security is usually easier, as keeping track of a lot of shares' passwords can quickly become cumbersome. However, if you have a relatively small LAN, setting up share-level security is simpler.
This explains it far better than I can, and detailed usage:
[Chapter 6] 6.3 Authentication Security
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