The iPhone, iPad and iPod touch are all great for watching videos, but Apple's devices don't support most video formats. In fact, the new iPad and iPhone are limited to playing H.264 videos if you want high resolutions: they can play MPEG-4 videos but only up to 640x480.
It's time-consuming to convert videos that aren't in exactly the right format, but the good news is that you don't have to. Third-party apps are capable of playing other formats, albeit by using the iPad's CPU rather than the graphics processor. That means it might eat up more battery power than using Apple's default video player, but in most cases it's worth it.
So what are your options? For most people, the best choice is an app called AVPlayer HD. Currently, it costs £1.99 from the App Store, which is a small price to pay for the convenience.
Other apps include Cinexplayer HD (£2.49) and OPlayer HD (£2.99). The best for you will depend on whether you want to stream the videos across your home network, or play them locally on your iPad.
For example, Cinexplayer HD will play files stored locally and supports Dolby Digital (AC-3) and .srt subtitle files. It can also stream MP4, DivX and M4V files - the developers say MKV streaming is coming soon.
AVPlayer HD supports a similar range of formats plus more subtitle formats and Dolby TrueHD. It will also let you transfer video files to your iPad via Wi-Fi as well as via a USB cable from iTunes. AVPlayer HD can also use the iPad, iPhone or iPod touch GPU for hardware accelerated playback, so long as the video is in H.264 format.
If you just want to stream files, try Air Playit HD, which is free on the iPad and can stream 320 formats over Wi-Fi or even 3G/4G connections, so can act as your personal cloud server.
If you have the latest iPad, iPhone or iPod touch, you should have no problems playing HD videos. On older devices, however, you might find you're limited to 720p or even lower resolutions. That's because the processor isn't powerful enough to handle higher-quality video.
With most apps of this type, the way to get videos from your computer to your iOS device is to connect the USB cable and launch iTunes. When your iPad or iPhone appears, click on it, then click on the Apps tab.
Scroll down to the section marked File Sharing. Click on the appropriate app in the list, such as AVPlayer HD, and then you can drag and drop files into the pane on the right. In iTunes 11, drag and drop support appears to have been removed, so click the small + button at the bottom of the pane to browse for files to add.
Once your files are transferred, you can disconnect the USB cable and watch your videos via the AVPlayer HD app. Note that these third-party video players cannot play video files stored in the native Videos app.