Computer files always have two parts: a name and an extension. They have two parts because the first one will tell both the computer and user which kind of information the file contains; while the second in which format it is stored.
File formats exist because not every programmer in the world did the framework to store information in the same way. In some cases we are talking about similar kinds of information, but in others, of something absolutely different. That's why giving every file a short description of what kind of information includes is mandatory for a computer. Yet it is useful for users too, as they will be able to identify the content too.
These days, file extensions are between two and four characters. Below you can find the most important ones:
aac, flac, m3u, mid or midi, mp3, ogg, snd, wav.
bac, bak, tmp, !ut
7z, ace, alz, bz2, bzip, bzip2, cab, deb, gz or gzip, package, rar, tar, tar.gz, tbz2, tgz, zip.
csv, dat, db, ods, odp, ppt or pptx, pps or ppsx, wk! or wk1 or wk2, xls or xlsx, xml.
cue, dmg, iso, isz, md5, nrg.
ai, bmp, cdr, dng, gif, jpg or jpeg, indd, mac, odi, pdf, pdn, png, pic, ps, psd, svg or svgz, tex, tif or tiff
doc or docx, latex, lrc, tab, tex, txt, wp or wpd, htm or html, dhtml, asp, php, rss.
3gp, avi, evo, hdmov, mov or qt, mpg or mpeg, ogv, rm
c, cc, c#, cpp, css, inc, java, pl, py, bat, com, exe, jar, vbs, msi.
eot, fnt or fon, otf, ttf, woff
To open any of the files above, you will need either to have the correct application or to convert the file into another file format.
On converting file formats
Given that software manufacturers never agreed to make all computer software work alike (they don't have to, either), and given that there is always room for improvement, now and then we will need to convert our files from a format to another.
Ideally, we would open our file with an application and we just "save it as..." the needed file format; yet there are situations in which this not so simple. Fortunately, there are already websites online which allow people to convert their files in a reasonably reliable way.
Some people wonder that, with so many file formats available, if one is better than the other. I would say no. Different file formats are made to meet different needs. Flac files, for example, have better sound quality than mp3 files; but are less compatible and larger, so people go for the mp3 files when exchanging them online, regardless of the quality loss. Something similar happens between a jpg and a png file. Jpg photos are very well compressed, but they do not accept transparency, so designers will use a png file now and then too. You get the idea.
What I do think is that there are file formats which are made to eventually supersede the previous ones. I mean the ones which are compatible among all computers, the ones which specialists call "cross-platform" formats. They are, in plain English, compatible between computer users around the world, regardless of whether they use a Windows computer, Linux or Mac. Something worth taking into account in a "global" world.
Which are these formats?
txt and odt for texts
svg for vector graphics
jpg and png for bitmap graphics
flac and ogg for music
ogv for vídeos
mxl for music score files
...just to mention a few examples.
Knowledge + Computers and the Internet