If you read through the GPS wiki, it should answer alot of your questions.http://en.wikipedia.org/wiki/Global_Positioning_System
Basically, there are 20 some Satellites going around the earth at all times, sending Signals down with their position compared to the earth. Your GPS device then finds as many satellites as it can, receiving information sent from the satellites, and triangulates your position via that information. (Time it takes to get a good fix on your location varies for each model)
I don't believe your GPS unit sends out signals at any time, it's only receiving. (I haven't read a whole lot, so I may be wrong)
And that's right, you will *NOT*
need a password or a login name, or anything of the sorts to use the GPS service.
They will also send back Speed, Altitude, Long, Lat, UTC (Universal Time), and some other items. (the precision of speed/and everything else depends on the receiver itself, and of course the reception you're getting and surroundings.)
GPS units are not trackable, unless you purchase GPS-tracking units, which send back the GPS data via some connection(such as cell-phone, wireless internet, satellites).
Most GPS units you're probably familiar with won't work with the Arduino.. But this is assuming you're familiar with cars / hand held versions. Basically, any GPS unit that has 5v TTL Output (which is TX/RX on the Arduino), such as, and not only:http://www.sparkfun.com/commerce/product_info.php?products_id=8936
This is the shield and the GPS unit used in the Tutorial I believe:
GPS unit fits this shield:http://www.adafruit.com/index.php?main_page=product_info&products_id=98
Totally different one, bread board capable, but spendy as well:http://www.parallax.com/tabid/768/ProductID/396/Default.aspx
(and about sending the data long distances, depends what data you need to send, how often you need to send it, and where it needs to be sent! and of course how far:D lol)