Magento Database Wiki
Use this Magento Database Wiki page to understand its structure.
EAV (Entity Attribute Value Model)
A significant proportion of Magento database is based on EAV model to maximize its flexibility. The EAV model makes it easier to expand the database, i.e. adding your own entities or attributes, without shaking it to its core. Its biggest drawback is complexity. You will probably have to come up with very complicated SQL codes to pull out some data in the format that you or your clients would understand better.
Before you customize Magento database, make sure you read through the EAV wiki page.