KDE Architecture - Starting programs
In KDE there are several ways to start other programs from within your application. Here is a short summary of your options with reasons why you should or should not use them.
fork + exec
You never want to use this unless you have a very good reason why it is impossible to use KProcess.
Other than the reasons about why you should not use fork + exec, system is also synchronous, thus it will block the application that run it.
You want to use KProcess class if you need to start a new process which needs to be a child of your process, e.g. because you want to catch stdout/stderr or need to send it data via stdin. You should never use this to start other KDE applications (unless your application is a debugger :-)
KToolInvocation::startServiceByDesktopPath is the preferred way to launch desktop (KDE/Gnome/X) applications or KDE services. The application/service must have a .desktop file, and you must know the name of that file. KToolInvocation asks the klauncher process, via DBUS, to start the program.
klauncher will wait until the application registers to DBUS if the desktop file specifies the X-DBUS-StartupType key. This allows to make DBUS calls to the application once startServiceByDesktopPath returns, without the risk of race conditions. Note that klauncher needs to know under which name the application will register to DBUS: by default that is assumed to be org.kde.binaryname, but you can configure this by setting X-DBUS-ServiceName in the .desktop file. If you don't, you will get an error from klauncher which will think the application failed to start, since the expected dbus name never showed up.
klauncher makes use of KDEinit for increased startup performance and lower memory usage. These benefits only apply to applications available as KDEinit loadable module (KLM).
Generic way to open documents/applications/shell commands. Uses startServiceByDesktopPath where applicable. Offers the additional benefit of startup-notification.
KRun can start any application, from the binary or the desktop file, it will determine the mimetype of a file before running the preferred handler for it, and it can also start shell commands. This makes KRun the recommended way to run another program in KDE.
KToolInvocation::invokeBrowser launches a web browser. The difference with using the more generic KRun on the webpage URL is that KRun has to determine the mimetype of the URL first (which, for HTTP, involves starting a download to read the headers), so if you know that the URL is an HTML webpage, use invokeBrowser, it will be faster.
More details: the problem with KRun for webpages is that it delays the appearance of the browser window, and if the user's preferred browser is a non-kde application like firefox then it has to start a second download while konqueror which can reuse the kioslave started by KRun. On the other hand if the URL might be an image or anything else than html, then KRun is the right solution, so that the right application is started.