In addition to the description of positions already given let me ad some things that I hope don't confuse anyone.
The positions that one tests in for groove welds is 1G,2G,3G,4,5G, and 6G. Each of these designations refers to a range allowed when taking a test.
In production, one welds in either the flat, horizontal, vertical or overhead position.
Here is a page I did a while back. My website has been having problems so it may not work.
http://weldingdata.com/testpositionsVSWeldPositions.htm Also, in reference to the pipe welding here is something I posted on another forum earlier that may be of help.
Don't underestimate the advantages that can be gained by the angle that the electrode is put in the stinger. Experiment with the position that allows you to travel the farthest with the least amount of effort.
Another KEY point I try to stress when I get the opportunity to teach somone is to "Start in a bind, and finish comfortable". When you start on the bottom of the pipe, be aware of what position you will be in after you travel an inch or two.
Keep your lenses clean and if you are struggling trying to see, take a look at how your lense is in relation to your eyes. You should be able to look straight out of your lens without moving your eyes.
The rest of the thread and other information on the forum may be of use too.
http://weldingweb.com/showthread.php?t=21900