You might have used a few different address book or PIM programs before. While MS Outlook has become a de facto standard of PIM, there are still a variety PIM program targeting different markets. Most of them can be comparable with MS Outlook.
Open Contacts is an advanced address book program; Outlook is a PIM with advanced address book functions. However, interfaces are provided in Open Contacts to communicate with other PIM programs such as Mozilla Sunbird and Google Calendar.
Open Contacts was designed for interacting with contacts you personally know; Outlook address book was designed for general purpose, including CRM. Though there are users using Open Contacts to manage business contacts for marketing activities, the functions and GUI were optimized with the assumptions that you personally know all contacts.
Open Contacts was designed to be open to evolution of communication methods with people, through unlimited dynamic data fields; Outlook is a typically traditional address book with static and limited data fields.
Open Contacts distinguishes between personal contacts and organizational contacts, and provide relationship links between contacts; Outlook with Small Business add-on may provide separated management for organizational contacts, however, navigating between contacts with relationships is still difficult.
Open Contacts was designed to be portable; Outlook can only be used as an installed program.