KMail is the email component of Kontact, the integrated personal information manager of KDE. Kontact is a personal information manager and groupware software suite for KDE. It supports calendars, contacts, notes, to-do lists, news, and email. It uses KParts to embed the various applications (KMail, KAddressBook, Akregator, etc.) into the container application.
Differences between “Kontact” and “KDE PIM”:
Technically speaking, Kontact only refers to a small umbrella application that unifies different stand-alone applications under one user interface. KDE PIM refers to a work group within the larger KDE project that develops the individual applications in a coordinated way. More Info »
In popular terms, however, Kontact often refers to the whole set of KDE PIM applications. These days many popular Linux distributions such as Kubuntu hide the individual applications and only place Kontact prominently.