OK, I did more data analysis and this is what I have so far:
Thanks JAG on the dt confirmation and for the u_h adn u_w values.
This is not a complete list of variables, but are the ones I have seen myself in my research. If you have more, please add them, but be careful not to share exact values of the variables.
If you have comments or feedback of my interpretation of the variables, please add them.
client - Unique ID representing the account that is showing this ad. This is the account that will get credit for the ad click.
dt - This is the time the ad was served in Unix Timestamp format with three numbers concatenated at the end. The time zone is UTC. I have no clue as to what the last three numbers at the end stand for. I thought microseconds, with three digits of microseconds, that seems way too extreme for Google to want to keep track of.
lmt - This also seems to be the time the ad was served, but without the last three digits that dt has at the end. Otherwise dt and lmt are equivalent.
alternate_ad_url - The publisher's specified url for the alternate ad if there are no ads for the visitor to be shown.
alt_color - The publisher's specified color to be shown if there is no ads for the visitor to be shown.
format - The publisher's specified format for the ad.
prev_fmts - Other ad formats coded in the HTML document that are higher to the top of the HTML code than the current format.
output - The output type for the ad. For me this always seems to contain the same value for my HTML ads, but this might be different for other ad formats like RSS.
channel - The publisher assigned tracking channels for the current ad.
pv_ch - Other tracking channels assigned to ads that are coded in the HTML document higher to the top of the HTML code than the current ad.
url - The location of the current page that is showing the document.
color_bg - The publisher's specified background color for the ad.
color_text - The publisher's specified text color for the ad.
color_link - The publisher's specified link color for the ad.
color_url - The publisher's specified url color for the ad.
color_border - The publisher's specified border color for the ad.
ad_type - The publisher's specified type of ad to run, such as text ads, image ads, or both. This is not specified for link ads.
loc - Rarely shows, but so far seems to be equal the the url value.
cc - No clue! This value seems to be a constant integer, but after reviewing my logs I did see this value change to a different integer. No idea way.
u_h - Visitor's screen height.
u_w - Visitor's screen width.
u_ah - Visitor's available window height (when you take the screen height and factor the size of the browser window, and space taken by browser toolbars).
u_aw - Visitor's available window width.
u_cd - Visitor's color depth of their screen.
u_tz - Visitor's timezone shift in seconds from UTC.
u_his - Not sure. This might correspond to how many times this visitor has seen ads on this page, without accounting for the number of ads on the page. However, that's just a guess.
ref - The referring URL the visitor was on immediately prior to viewing the publisher's page with the ads.
qsrc - Not sure. Seems to be an integer that shows if the ref value is from specific domains.
o - Not sure. Seems to be an integer that shows along with qsrc.
l - Not sure. Seems to be a text string that shows along with qsrc and o.
ptnrS - Not sure. Seems to be a text string that shows if the ref value is from specific domains. (Does not seem to be related to domains corresponding to qsrc, o and l values).
searchfor - Words searched by visitors. Shows with ptnrS. These terms are not visible in ref value.
u_nplug - No clue. Rarely shows. Seems to be an integer.
u_nmime - No clue. Rarely shows. Seems to be an integer.