You may or may not be very aware of file types.  Computer operating systems have evolved to the point where it's not THAT necessary to know.  When sharing documents, however, it becomes VERY valuable knowledge.

What is a file?

Files are discrete chunks of data on a hard drive.  Nothing more than 1's and 0's, really.  They're more like ideas than real objects, yet we get so very concerned about them.  Funny, in a way.

There are different types of files.  Some are text documents, some are pictures, some are videos, some are music, and some are programs.  Most programs generate and read special types of files.  Some types of files are so popular that several programs can read them.

Computers know what a certain file is by its extension, which are those three or four letters after the period in a filename.  In the file "basename.doc", the .doc is the file extension.

Here's an overview of popular file types and extensions.

Documents

.doc This is the standard document format, created by Microsoft Word.  Microsoft Word generates and reads these files.  Open Office can also generate and read these files.

.docx This is Microsoft's newer format, which is created by Office 2007.  Older versions of Microsoft Office can't open this format.  If you use Office 2007 and want to share a file, please use compatibility mode.

.rtf Sleek and sufficient for most needs, this is the rich text format.  This is not often used.

.txt When you just want the text, this is the text-only extension.  

Spreadsheets

.xls The standard spreadsheet format, this is generated by Excel.  Open Office can also open and write these files.

.xlsx Excel 2007.

Presentations

.ppt Powerpoint!

.pptx Powerpoint 2007  :(

Pictures

.jpg JPEG pictures, these are very common.  

.gif GIF pictures, also common.

.png PNG pictures, not as common (but very good)

.bmp Very large pictures.  

Music

.mp3 You must have heard of these by now?

.ogg An open-source alternative to MP3

.wav An UNCOMPRESSED file format.  These tend to be VERY LARGE.

Movies

.avi AVI movies. Pretty common

.mpg MPEG movies.  Also common

.flv Flash movies.  This is what you get if you download a youtube video.  Can be opened with VLC media player.

Other

If you're still putting your grades in one of the gradebooks in this picture, sthen I'm afraid you're doing both yourself and your students a great disservice.

Now, I don't mean to push needless technology on teachers.  Technology used as a toy is a waste of time.  Keeping your grades in a paper gradebook, though, goes against the grain of all that is well and good with the world.  Here's why.

First, grades aren't necessarily straightforward.  You might have one scale for one group of kids, and another scale for another, but maybe you want to add extra weight to one assignment over another to play on the strengths of your students.  You could work mathemagic in your book to accomplish this, or you could enter a few numbers into a few boxes and have your grades automatically calculated.

Second, students and parents love to keep track of grades and assignments once they've tried it.  Students can see how they're doing, and parents can see if their children are slacking.

Many schools and districts now mandate online gradebooks, so there's never been a better time to become proficient in their use.  I'll cover just two here.

Engrade

This is my favorite.  It's simple, sleek, and it just works.  Setting up the classes is a bit arduous as you need to input a random number for each student.  Setting up classes is easy, as is copying class settings.

Engrade has recently added wiki and flashcard functionality.  It deviates from their core product, yes, but it sure is cool. 

LearnBoost

This is a new player in the field, and it's receiving some high marks.  I played with it a little bit and I found it highly functional and easy to use.  Entering students is a breeze, as is setting up classes.  Try their tour.

Google News is out of beta and it has a new look.  Now you can localize the news, create a custom news portal, and sort your sources.  You can even add custom sections, including Social Networking.  This is kinda like an RSS reader, but you just select the topics, not the specific sources.  This information will also be as vetted as you can expect public news to be (caveat emptor).