A slug pub is exactly what you might suspect it is: an open beer bar for slugs and snails. However, a slug pub operates a bit differently from a human-oriented pub. The goal of a slug pub is to eradicate such creatures from the garden, ensuring that they do not damage or kill valued plants. Several garden supply companies make slug pubs for gardeners, and making your own is fairly easy.
Slugs and snails are attracted to the aroma of beer, so a slug pub can be a very effective form of bait. There are several theories as to why slug pubs work. In the first case, many gardeners suspect that slugs become acutely dehydrated when they consume beer, in particular if the beer is salted, so they die. Others believe that slugs and snails die in slug pubs, especially when they are designed to make escape challenging.
Beer has been used for quite some time as a natural method of pest control. The simplest slug pub is simply a saucer filled with beer and deposited near some particularly tempting plants, with the goal of drawing the slugs and snails off. More complex versions are enclosed, buried, or designed to be concealed so that they do not become unsightly, although it is important to check them on a regular basis to remove victims. Enclosed slug pubs can also entrap more slugs, as slugs and snails often gravitate towards dark, damp spaces.
There are several advantages to using a slug pub over conventional slug bait. In the first place, slug pubs are nontoxic, making them appealing for gardeners who wish to grow organically. The use of slug bait is also dangerous for neighborhood pets and animals which might feed on the slugs, as the poison could potentially be passed higher up the food chain, sickening or killing harmless or even beneficial animals. Also, as some advocates point out, while slugs may die in slug pubs, at least they die drunk, and presumably therefore happy.
For slug pubs to be effective, you need a lot of them. It's a good idea to start around plants which are particularly infested with slugs and snails, and to work your way out through the garden. It is also important to clean out your slug pubs frequently, as otherwise they can become rather gruesome. Incidentally, slugs do not seem to have any brand or type preferences; this wiseGEEK author uses a locally-brewed stout, but presumably any sort of beer would work.