As mentioned last week, I've declared August to be dim sum month! I'll be posting two of my favourite dim sum dishes each week this month, so today I thought I'd give a quick overview of what dim sum is all about. (Update, August 2011: here's a list of all the dim sum dishes I've ever posted about.)
Essentially, a dim sum meal consists of a number of small dishes of mostly savoury, mostly snack-like food, accompanied by large quantities of Chinese tea. In the UK, the term "dim sum" is used both for the type of food served, and for the occasion itself. Elsewhere, "yum cha" (literally "drinking tea") is a more common term for the act of going out to eat dim sum. A dim sum meal may last less than an hour — a good few of my dim sum outings have been on weekdays with the London Perl Mongers, so people need to get back to work — or it may be slow and leisurely. I think my longest dim sum lunch ever lasted just over three hours.
The Chinese characters for "dim sum" are 點心, which in Mandarin is diǎn xīn. However, dim sum is a solidly Cantonese tradition, and so the names of dim sum dishes, as well as the overall term "dim sum", are almost always transliterated with Cantonese spellings and pronunciations. (Note: I'll still be giving the Mandarin/pinyin for the dishes I post about this month, since many of the characters also appear on non-dim-sum menus.)
Dim sum is a breakfast/brunch/lunch meal, not only out of tradition, but also because it involves plentiful tea-drinking — evening drinks like wine and beer don't really go with this sort of food, and you probably wouldn't want all the caffeine from the tea in the evening. Like 火鍋/huǒ guō/hotpot, it's a highly social occasion, and the more people you have along the better. All dishes are shared, and there's no concept of starters or mains — it all arrives as and when it's ready, and it's fair game for whoever wants it. Some items (e.g. dumplings, pastries) come in multiples of three or four, and others (e.g. rice, chicken feet, tripe) come in a single bowl which you can serve yourself from as required.
Generally when I go out for dim sum, I find things go smoothest when one person puts themselves in charge of the ordering. Having one person in charge means you're more likely to get a nice spread of dishes — ideally, you'd have a mix of steamed, fried, and baked dishes, with a variety of ingredients. Often, instead of ordering by telling a member of staff what you want, you'll be given a paper menu with tickyboxes on which you check off the required items (photo). Some restaurants even still have dim sum carts, which can be fun — these are heated trollies filled with steamer baskets of dim sum, pushed around the dining room by various waitstaff (photo). As they pass, their proprietors will show you what they have, and you just tell them which items you want. This works nicely in terms of getting food on your table without any waiting, but the quality of the food can suffer as it sits around in the trolley. (Londoners: trolley dim sum is available at Chuen Cheng Ku and the New World.)
The most important thing to bear in mind when trying to find good dim sum is that while some restaurants will have a dedicated dim sum chef (or team of chefs) to make the dishes from scratch, other places just buy the items pre-made and frozen. If you see a Chinese menu that includes ten or fewer dim sum dishes, it's a pretty good bet that these aren't being made in-house [see footnote]. For proper dim sum, you want to look for places that have entirely separate dim sum menus with dozens of items — and bear in mind that it's usually different chefs in charge of dim sum and in charge of the regular menu, so a restaurant with indifferent à la carte may well do good dim sum, and vice versa. Take a look at Mr Noodles' post on old school dim sum for some more hints. Dim sum is normally only served until late afternoon; common hours in London are 11am-5pm.
One aspect of interest in the UK at the moment is the rise of the "dim sum chain" (example). Unlike the traditional dim sum places, these chains feature cocktails, snazzy decor, cool music, dim sum served right through to the end of the evening, and polite (even obsequious) service. Prices tend to be higher than in the old-school dim sum joints, and the menus can occasionally be frustrating if you're already familiar with the usual terms and would like to know exactly what something advertised as a "pork dumpling" actually is. Having said that, they're not all bad, and they can work well both as an accessible introduction to dim sum and as an occasional diversion for those tempted by the terribly aberrant practice of eating dim sum in the evening rather than at lunchtime :)
Speaking of aberrant practices, going out for dim sum on one's own can be frustrating, since it's hard to get a nice spread of dishes without being forced to over-order. If you're lucky, you'll find a place that offers a dim sum taster platter and does dim sum that's actually good. One such place is Pearl Liang, near Paddington Station in London; their nine-piece dim sum platter is shown below.
Footnote:  There are two exceptions to the "short dim sum menus are a bad sign" rule.
The first is if they mention hand-made dumplings. Chinese dumplings per se are not purely a dim sum item, though the non-dim-sum version is more rustic and less delicate. On most menus these are listed with the rice etc, but sometimes they're set aside in a "dim sum" section, presumably for marketing reasons. They aren't really dim sum, but they can be good, particularly if you see the character 手/shǒu/hand in the name, for example as 手工水餃/shǒu gōng shǔi jiǎo/"handmade boiled dumplings". Dumplings that are offered in large quantities (≥10) at cheap prices are likely to be hand-made in-house. Some menus may list both the pre-made frozen items and those made in-house (example — note the relatively expensive steamed items, in contrast to the 30p-per-piece pork/chicken "Chinese Dumplings").