Dolphin is the new (trimmed on usability) file manager of KDE4. Dolphin isn't a one-does-it-all application like Konqueror. Instead, it's limited to file management. Dolphin can connect with other KDE programs via KParts and KIO-Slaves. For example, KDE4 Konqueror uses a Dolphin-KPart to offer file management.
Dolphin provides various view modes. For example, a symbols mode with optional previews and various list views. The view mode can be modified and your personal data can be categorized and sorted. Detailed information about a file can be displayed as a pop-up or in a separate information field. In the future you will be able to control your semantic desktop with Dolphin.
By default Dolphin displays only a minimal number of icons in the working bar, which can be modified like all KDE applications. The address bar shows navigation paths by default. Alternatively, you can switch to a text based navigation bar.
Dolphin can also be run in 'Super User' mode, which runs the application with root privileges. The user must enter the root password to launch the application in this mode.