Since this seems to be the main FAQ it seems like a good place to start.

#In! represents the number of people that came to the TWC list from your comic, hence the number of votes you have. #Out! is how many people went from the list to your comic, i.e. clicked on your banner or link on the TWC list to get to your site.

The total column represents the ins and outs of comic since it entered the list, whereas the In/Out column shows the ins and outs for the current month, and it is this which determines your place in the list. At the start of each month this is reset for all comics, which often means that some of the smaller comics might be able to hold onto high ranks for a short while.