From MEPIS Documentation Wiki
The data storage access mechanism for all PIM (Personal Information Manager) data in KDE SC 4. One single storage and retrieval system allows efficiency and extensibiliy not possible under KDE3, where each PIM component had its own system. Note that use of Akonadi does not change data storage formats (vcard, iCalendar, mbox, maildir etc.) - it just provides a new way of accessing and updating the data. The main reasons for design and development of Akonadi are of technical nature, e.g. having a unique way to access PIM-data (contacts, calendars, emails..) from different applications (e.g. kmail, kword..), thus eliminating the need to write similar code here and there. Another goal is to de-couple GUI applications like kmail from the direct access to external resources like mail-servers - which was a major reason for bug-reports/whishes with regard to performance/responsiveness in the past.
Source: KDE Glossary