If you are using Mac OS X 10.7 Lion or 10.8 Mountain Lion, you can easily convert video files to iPhone/iPad compatible format right on the desktop or from any Finder window using the built-in video encoding feature.
To convert a video file on Mac, just right-click on the file and choose Encode Selected Video Files. Then you will be able to convert the video to iPhone/iPad video, and even convert 1280p/1080p video to lower resolutions like 720p and 480p.
Larger videos and movies will take longer to convert while shorter videos can be very quick.
Once the movie is in the iOS compatible format, simply transfer to the iPad or iPhone with the sync of iTunes. Once a video has been copied over to the iPad/iPhone it is watchable through the Videos app. Or you can transfer video to the iPad or iPhone without iTunes (using Wireless Transfer App).
If you want to convert video to audio, just choose Audio Only from the Settings pull-down menu. The video files will be converted to m4a audio tracks.
Note: If your Mac is missing the Encode Selected Video Files menu option, it is easy to toggle it.
Go to System Preferences -> Keyboard -> Services and tick the Encode Selected Video Files option.
Also tick the Encode Selected Audio Files if you want to convert audio to m4a which can then be turned into ringtones and text tones.
Note: Though this method is quite simple, it cannot process video formats like MKV, WMV, MTS, M2TS, etc. In this case, you can still use Video Converter App to help you get these video files transferred and converted to iPhone/iPad compatible videos.