A file's format - the way that it's saved and encoded - determines what you can do with it and which programs will open it. You can check its format by looking at the file name's extension. Think of each format like a different language, with some only understood by specific platforms and applications. To access any given file, you may need to convert it to a new format. For example, something created on a macOS word processor might not work on your Windows 10 computer. Or a website that asks you to upload a photo might insist that it be a specific type of file. Whatever quandary you encounter, you can usually hunt down an app that will help.
Frequently, the application you used to create the file in the first place will let you save it as something else. If you're viewing a document in Google Docs, for example, you can click File and Download to bring up a list of formats to which you can convert the page. These include Microsoft Word, PDF, and plain text. The desktop programs Word and Apple's Pages offer a similar selection of formats. It's the same with image editing programs. Most of these applications let you save your pictures as a variety of file types, so you can pick the appropriate one. Export an image from Photos on macOS, for example, via File and Export. It will let you choose between JPEG, PNG, and TIFF formats. If you know you'll want to access a given file using multiple programs, you can save it in several formats to begin with. Of course, you won't always have access to the program that originally created the file. In that case, you'll want to use a conversion program. The program you choose will depend on the type of file you're tinkering with.
For your video conversion needs, you can't go wrong with the free programs Handbrake and VLC. Handbrake (for Windows, macOS, and Linux) takes video files from almost every imaginable format and converts them into any other popular video type. It's simple to operate and even lets you pick from standard pre-set modes in order to, say, optimize the videos you're converting for an iPhone. When you're more comfortable with the software, you can dig deeper into the settings for the codecs - the exact standards by which the video files are saved. Beyond that, Handbrake can rip videos from DVDs and supports subtitles and chapter markers in your conversions.
You can find even more free programs to convert your photos, so we've selected our top three picks. XnConvert (for Windows, macOS, and Linux) lets you resize pictures, change the color depth, and even add text as you're converting your photos. And if you're in a hurry, you can convert whole batches of images at once.
The simple and straightforward Free Audio Converter from Freemake (for Windows only) is another good choice. With this application, just drag your audio into the program window, choose your output format from the list at the bottom, and you're good to go. It supports common audio formats such as MP3, AAC, WMA, OGG, and FLAC. Finally, if you want to take more control over the audio as you convert it, opt for the free audio editor Audacity (for Windows, macOS, and Linux). In addition to a full suite of editing tools, it gives you the option to convert your files between various popular formats. To do so, open the track in question, hit the File menu, and head to the Export option.
Compared with music and movie files, documents seem relatively simple. But converting between document formats can be trickier, because many of them - the Word document format from Microsoft, the Pages document format from Apple, and so on - are proprietary. Because of its finicky format, you should, wherever possible, try using a document's native program to open it and convert it to something else. If you don't have the right software to do so, consider falling back on a web app. You can access basic versions of both Microsoft Office and Apple iWork online, allowing you to use them from any browser for free.
If you still need a dedicated tool, we like the free Doxillion Document Converter Software (for Windows and macOS). It supports Word, HTML, PDF, and other common document formats. And it's easy to use, handling everything through a single one-screen interface. Hit the Add Files button to build a list of files, then set your output options in the pane underneath. Finally, the free online document converter FileZigZag can competently handle a variety of file types, including Microsoft Office formats and PDFs. Like Zamzar, it makes you provide an email address in order to receive a download link for the converted file. But other than that, it's straightforward and easy to use, and you can run it from any browser window.