A beer tasting is an opportunity to sample a wide variety of import and microbrew beers. Chances are, with all the vendors that usually frequent beer tastings, you'll have a lot of beer to consume. Unless you happen to be Hank the Tank, you will probably experience the full effects of inebriation.
Remember though, a beer tasting is a gathering of gentlemen (and ladies) and your behavior should in turn be gentlemanly. A beer tasting is not the place to whip out your homemade beer bong and pound an Irish Stout. Save that for another night. You can also get some ideas about the tools like Bar Tools, Cocktail Bar Supplies via Bevtools.com online and try it in your home by simply buying it.
Also, don't worry too much about what you wear. Jeans and a t-shirt is perfectly acceptable at most tastings. If the tasting is open to the public, jeans and t-shirts are fine. If you are invited to a private tasting (you lucky dog), dress a little more formal, khakis and a polo should do.
Oh, and another thing- this isn't a wine tasting. Don't spit your beer out. If you don't like your beer, swallow what's in your mouth and pour the rest of the beer into a pot on the table. A word of warning, be careful about pouring your beer out – there have been reports of confrontational Brewmaster's pestering unsuspecting beer drinkers for pouring out their beer at the Brewmaster's table.
The list is simply a sheet of paper that beer coinsures carry with them when they go to a tasting. Beers that the connoisseurs particularly enjoy make it onto the list. At the end of the night, the connoisseur will have a list of beers that he will buy on his next beer run.