File Formats

When data are saved to a file, they must be organised so that programs can re-read and correctly interpret the contents of the data. There's not much good saving a digital photo if your web browser or graphics editor can't read it.

There are standard file formats used to ensure that different types of data can be reliably saved and read, often by a variety of programs: not just the program that saved the data file.

For some data types, there are different formats to choose from. Selecting the right one for a particular job can require some judgement. The file format is indicated (at least on Windows platforms) by the filename extension: the letters after the last dot in the filename (e.g. "invoice.doc", the extension is "doc" which indicates it's probably* a Microsoft Word document).

Most formats sharing a purpose (e.g. DivX and WMV) can be converted from format to format, usually with a significant cost in terms of quality loss. Uncompressed or lossless formats do not suffer this problem.

Let's look at common file formats for saving common data types, and factors to consider when choosing amongst competing formats.