It's never been easier to compare hotel prices online, but it's vital to know what you're comparing. Hotel listings can sometimes seem to be in their own language, but once you've mastered a few terms you'll find it easier to know what you're getting.
Hotel: locals already know this, but it can be confusing to visitors -- a "hotel" in Australia isn't necessarily a place to stay, such as the Windsor Lodge Como. For historical reasons, a hotel can also mean a pub.
Room positions: to most of us, if two rooms are adjacent they're directly next to each other. In a hotel, though, it can also mean that they're across a corridor from each other. Two rooms next to each other are adjoining, but that doesn't mean that you can go straight from one to another; if you could, the rooms would be interconnecting.
Bed sizes: a single room is for one person and contains one single-sized bed. A double room contains one double bed and can sleep one or two people. If two people who aren't a couple are going to be sharing a room, they'll probably need to request an extra bed. This setup is called a twin room or, informally, a double-double. These beds can be of any size; check if you're unsure. A triple room often has one double and one single bed; they're popular among parents travelling with children.
Amenities: a concierge can give you information on the city you're in, but might also provide other services such as helping you book hard-to-get event tickets. A butler service takes this one step further, assigning you a personal assistant who will handle everything from unpacking to cooking to administration; this level of service is rare. Business centres provide equipment such as printers and copiers for business travellers. Some even offer secretarial services, although again this is rare.
Rates: all-inclusive is just what it sounds like; your rate includes not only accommodation but meals; however, "all" can be a loose term; most but not all hotel services will be included, so check before using them. Half board (or, rarely, American) includes some meals but usually not lunch.
Behind the scenes: C&C refers to "conference and catering" or event planning; C&E is the same thing. If you're unlucky enough to be in an overbooked hotel -- one where there have been too many reservations -- you may be bumped or moved elsewhere. You usually won't run into this kind of terminology unless you're in the travel planning trade.