Many of Konqueror's features can be used regardless of what you are viewing: for example, multiple tabs, the sidebar, window frames and bookmarks. This makes it not only a powerful web-browser, but also a powerful general-purpose application.
Using tabs in Konqueror is similar to using tabs in many other KDE applications. You can create a new tab by going to the, "File" menu. You can find ways to interact with tabs by right-clicking any tab you currently have open. You can also move tabs by clicking with the middle mouse button and dragging them across the tab bar.
Frames allow you to view multiple locations, documents or websites in a single window under a single tab by splitting the window. This can be used e.g. to view multiple web pages at once, to copy files from one location to another (either from one folder to another or from a remote location like an FTP site or a shared network folder), view or compare two documents at the same time and for many other possibilities. To use frames, either go to the Window menu or right-click on the status bar at the bottom of the window and select either, "Split view left/right" or, "Split view top/bottom". You can also create or remove frames using the short-cuts displayed in the Window menu for each action. You can control each view by clicking on it and interacting with it as usual.
Since Trolltech ported Webkit into their toolkit in Qt 4.5, there has been a lot of discussion about replacing KHTML with Webkit in Konqueror. However, since KHTML is still being actively developed and is a requirement of KDE's core libraries, it will not be replaced by Webkit in either Konqueror or any application that allows previews of HTML files. A Webkit KPart is under development by the project, "webkitkde", but it is in development stages and is not recommended for widespread use. Also a problem is that Qt's version of Webkit is somewhat outdated and, though various web browsers are being developed with Qt's Webkit (arora and rekonq, for example), none of them are recommended for daily use yet.
Konqueror uses the Dolphin KPart for file management, giving it all the abilities of Dolphin. It includes many configuration options under the Settings menu --> Configure Konqueror... --> File Manager configuration dialogue. Changes applied here will affect Dolphin and vice versa. For all of Konqueror's file-management capabilities, check Dolphin's page.
Konqueror is also a universal viewing application, thanks to its ability to embed audio, video and document viewers.
Starting KDE 4.2, you can change the default file manager through System Settings --> Default Applications
This forum page deals with improving the display of GMail within Konqueror
File manager, File browser, File management, web browser, Internet, file viewer, embedded preview, KPart, KIO