This. This is lacing.Lacing is simply the residual effects of having good quality head. You heard me. Get your funnies out now, because I'm going to be saying 'head' in increasingly double-entendry ways from now on. Or don't get your funnies out; I don't care.
When a beer is well made, having a full body and a good balance of the proper ingredients (hops and yeast in particular), it can retain its head for most of the time it takes to drink the pint. Think of an Irish stout, like Guinness. If you've ever had Guinness on tap (and even sometimes in a can), there is almost always a thick, creamy head that sits respendently on top of the body and remains throughout the pint. Even as you're finishing, there's usually a thin layer of foam hanging around to the bitter end. Whenever you take a sip, some of the head sticks to the side of the glass in a reaction scientists refer to as "science". As you work your way through the beer, more and more head sticks to the glass, sometimes cascading in sheets, other times patchy like sea foam. The criss-crossy effect on a glass is known as lacing, and for beer geeks like me, it's very important.
Like I said before, a thick head is a good sign (*but not an essential sign!*) of a well-made beer. Longer lasting heads mean more lacing. Having lacing on your glass can also have the effect of giving your beer a creamier quality, guaranteeing that each sip will include a little bit of head. Poorer quality brews, like your standard macro lagery fare (Canadian and Budweiser) are usually thinner, and not up to snuff. Their heads having disappeared within about 30 seconds, they just sort of sit there in the glass, looking quite a bit like pee. Alcoholic pee, but pee nonetheless. In some ways, lacing is really just an aesthetic thing: it's a thing of beauty to drink a pint that produces sheets of lacing up and down the glass.
However, not all brews produce good lacing, whether it be their style (pilsners are tough to get good lacing going) or their internal chemistry, and that's okay too. Hence my asterisked disclaimer above. Indeed, just because your beer doesn't have head doesn't mean its a poorly made beer. For example, Coopers Sparkling Ale, a pale ale from Australia, didn't produce a lot of head, which might be a result of the beer having (deliberate) residual yeast sediment in the bottle. It was a tasty beer, but it had nearly no head to speak of.
There is another reason why a particular beer doesn't produce head/lacing, and that's entirely yours or the bartender's fault. A poorly washed or rinsed glass impedes the ability for head to stick to the side of the glass, again the result of "science". You're not getting any lacing because there's too much crap on the glass (usually dishsoap or dust), and thus you're missing out on the lacing experience needlessly. (FUN FACT: ever poured a beer and noticed a lot of carbonated bubbles leaving from one particular area of the glass? This means that your glass isn't completely washed, and whatever is stuck to the glass is reacting with the beer to produce excess carbonation at that point. Neat!) Therefore, whenever I work at the bar, I always try to ensure that every glass is well-rinsed with our nifty glass-rinsing device before pouring, and I always do the same at home. It really makes a difference.
So, to recap: Lacing is always a good thing, but not having lacing isn't necessarily a bad thing. Some beers just won't produce the lacing effect. But lacing comes from two sources, so keep up your end of the bargain by keeping your glassware clean and rinsed. Like I always say, you can never rinse your glass too much!