Using this table list as a blog equivalent of an ER diagram:

relationships
acl_roles
acl_actions
leads
contacts
accounts
opportunities
cases
notes
email_templates
emailman
calls
emails
meetings
iframes
tasks
users
currencies
tracker
import_maps
files
users_last_import
config
upgrade_history
vcals
bugs
versions
releases
feeds
project
project_task
roles
email_marketing
campaigns
prospect_lists
prospects
documents
document_revisions
fields_meta_data
schedulers
schedulers_times
inbound_email
campaign_log
dashboards
campaign_trkrs
saved_search
user_preferences

List of relationships:

inboundemail_autoreply
usersignature
accounts_bugs
accounts_cases
accounts_contacts
accounts_opportunities
acl_roles_actions
acl_roles_users
calls_contacts
calls_users
cases_bugs
contacts_bugs
contacts_cases
contacts_users
custom_fields
email_marketing_prospect_lists
emails_accounts
emails_bugs
emails_cases
emails_contacts
emails_leads
emails_opportunities
emails_project_task
emails_projects
emails_prospects
emails_tasks
emails_users
files
linked_documents
meetings_contacts
meetings_users
opportunities_contacts
project_relation
prospect_list_campaigns
prospect_lists_prospects
roles_modules
roles_users
tracker
users_feeds