Can I modify Simple CRM or is it a fixed structure?

Simple CRM has, like any CRM software, a basic structure.

This structure can be modified by you, via the creation of custom fields named Interests
(see info on http://www.simple-crm-online.com/test),
Interaction templates
(see: http://www.simple-crm-online.com/test) and Email templates (see: http://www.simple-crm-online.com/test).