Here's what I advise to our clients about SugarCRM and how to store data:
Leads
These are completely cold contacts -- people and companies you have never contacted. Once you have initiated contact and they have requested more information, or expressed some level of interest, convert them to a contact/account.
Contacts
These are the individuals that link to accounts (they don't have to be linked, but to get the most out of SugarCRM, I recommend it). In a B2C situation, you can make an account with the same name as the contact, for example.
Accounts
This is much like a central point that you can cross-link all information back to. Warm leads, customers, suppliers, etc should all have accounts that let you review your history of intereaction with them.
Naturally, everyone's situation will be different, and my suggestions may not perfectly meet your needs, but it should give a good starting point to better understand how data is held in SugarCRM.

Bookmarks