While the iPhone is basically a tiny computer that fits in your pocket, it doesn't have all the flexibility of your laptop or desktop. For instance, you can't install any program you want on it—you have to use programs from the App Store (or Cydia, if you've got a jailbroken iPhone). If there's not a program in the App Store to do what you want, you're stuck.

Apple built support for many common kinds of files—Word, PDF, MP3 etc.—into the iPhone.

But that support varies. For instance, you can view a Word document, but not create a new one.

This list covers some of the most common filetypes and provides options for using them on the iPhone.

AAC
Kind of file: Audio
Extension: .aac
Use: Built-in iPod app

Audiobooks
Kind of file: Audio
Extension: varies (often .mp3, .aac, or .aa)
Use: Built-in iPod app; third-party apps such as Audible

AVI
Kind of file: Video
Extension: .avi
Use: Third-party apps, including CineXPlayer

eBooks
Kind of file: Text
Extension: varies
Use: Apple iBooks app; Third-party apps, such as Amazon Kindle, Barnes & Noble nook, and many more

Excel
Kind of file: Spreadsheet
Extension: .xls
Use: View Excel files via built-in iPhone Mail or Safari apps; third-party app Documents To Go allows view/edit/create; Apple's Numbers app for iPad allows viewing and editing; various other third-party apps

Flash
Kind of file: Web audio and video
Extension: .swf, .flv, .fla
Use: There is no way to access Flash files on the iOS, as Apple is blocking the Flash player for iOS

Images
Kind of file: Image
Extension: .jpg, .gif, .tif
Use: Built-in Photos and Safari apps; various third-party image viewing apps

MP3
Kind of file: Audio
Extension: .mp3, .m3u
Use: Built-in iPod and Safari apps

PDF
Kind of file: Text
Extension: .pdf
Use: Built-in Mail and Safari apps; third-party apps such as GoodReader, PDF Reader Pro, Readdle Docs, MyPDFs, and PDF Expert

PowerPoint
Kind of file: Presentation
Extension: .ppt
Use: View in built-in Mail app; third-party app Documents To Go allows view/edit/create; Apple's Keynote app for iPad allows viewing and editing

Windows Media
Kind of file: Video, Audio
Extension: .wmv, .wma
Use: Third-party apps like WMV Player, SharePlayer, yxPlayer

Word
Kind of file: Text
Extension: .doc, .docx
View in built-in Mail app; third-party app Documents To Go allows view/edit/create; Apple's Pages app for iPad allows viewing and editing

VLC
Kind of file: Video
Extension: .vlc
Use: Third-party apps like VLC Media Player and CineXPlayer

Other Video
Extension: varies (includes .avi, .mov, .mp4)
Use: Built-in iPod, Safari, and YouTube apps