You need to copy the required dll's in the same folder with your application.exe

You need at least two dll's for MingW runtime:
mingwm10.dll and libgcc_s_dw2-1.dll
you can find this two in the QT_INSTALL_PATH\mingw\bin

And some (depending on what modules you use) for Qt runtime, for example: QtCore4.dll and QtCore4.dll (these are for release build of your application, those for debug have a d in their name and are bigger because they have more debugging information) you can find those in QT_INSTALL_PATH\qt\bin
Note: the dll's from QT_INSTALL_PATH\bin won't work with your application.

Also on your developing system you can add the two paths to the Environment variables Path and on your system you don't need to copy the dll's, you will copy them only when you deploy on another system