Users – Are users of application that can login and perform actions. Records can be assigned to them and only they see assigned records.
Contacts – Are persons, buyers of goods & services. Contact can have multiple-emails, can be in campaign, can have call-log (in call center case), deals, tickets, orders, invoices, assets (physical or virtual), quotes, documents, comments, attachments.
Accounts – Are companies that can collect contacts. Accounts can have tickets, call-log, deals, quotes, orders, invoices, documnts, campaings, service contracts, assets, comments, attachments.
This is multi-purpose crm you can use this as crm for marketing, crm for online shop, crm for lead collecting, call-center etc.