[quote=&quot;MikeAnime&quot;]what do you mean tag a photo?[/quote]
If you have to ask it probably won't affect you :P

Jpeg and other image file formats allow metadata to be added.  Comments basically.  The best damned image viewing application in the world can show you this metadata and believe me it aint fuckin' PAINT!  There are basically two types of metadata that I know about, EXIF metadata and IPTC metadata.
http://www.exif.org/
http://www.iptc.org/pages/about_main.php
EXIF data is usally written to a file when a digital camera creates the file or by another program when it feels like it.  Digital cameras will record the camera's settings and some other stuff when the image is shot... that way when someone brings me an image and tries to blame me for the way it looks I can just show them their EXIF data and say 'wait one minute Mr. Douchebag, what photography class or book told you it was okay to shoot at 3200 ISO indoors with poor lighting?!  All righty then, give me my money and learn how to use your camera or just give up on being a professional photographer.'
IPTC data is used more for organizing and cataloging images.  Keywords, descriptions, bylines, whole buncha crap is specified in the IPTC metadata.  Many programs write info to this area also but most of the time it's something you would put in yourself somehow.  Typically the only images I ever see that make use of the IPTC data come from a pro photographer who's put their contact and/or copyright info into it.  
This metadata has been around for years.  The general public's lack of knowledge about it is sad IMO.  Nobody cares about their images like they should tho... it's one of the reasons why I'll be out of a job soon... I can deal tho.  I have to.

The thing about RAW files - forget it.  You'll likely never see a RAW image file unless you have a camera (usually only digital SLRs do this but not always) that shoots and saves in a RAW format :)

The next time you see some artist on TV or in a movie or whatever (whether fictional or documentary) using an Apple OS instead of a Windows OS you'll no longer wonder why :P  This is just one more reason not to use a Windows OS if you're much more an artistic than techy/geek.

Then again there's also the other end of the spectrum of Apple users... the ones who don't like windows because it's TOO HARD to figure stuff out on.  In my world those 2 groups (idiots and artists) make up the largest percentage of Mac owners.  Not all artists are idiots and most idiots aren't artists... and that's not to say that there aren't any artists or idiots (haha there'd be no AOL if that were the case) using Windows.  Yeah.

[size=7:cbaf8319cc]*DISCLAIMER: I have never used MSPaint or the retarded Picture/Fax viewer thing so if it does indeed show any metadata I don't know or care.  Paint sucks.  Get irfanview already.  It's only been kicking every other app's ass for speed and simplicity for 10 years or so now.[/size:cbaf8319cc]