The guide below is by necessity a generalization, as star ratings are awarded by each country according to their own rules, and the difference between a 3-star and a 4-star may be something as obscure as having a minibar in each room. It's also worth noting that star ratings are often 'sticky', in the sense that once awarded they're rarely taken away: a four-star built last year is probably still pretty good, but a four-star opened in 1962 and never renovated since may well have turned into a dump.
Note also that the ratings are weakening as marketers misuse them.
Six and seven-star hotels
The notion that a hotel can be six or seven stars is a joke among travel professionals since most respectable hotel rating systems do not give out a rating higher than five stars. The consensus is since so few hotels really can achieve the five star rating then there shouldn't be a rating higher than five stars.
An example of a claimed "seven star" hotel is Dubai's Burj al-Arab. It's certainly one of the most luxurious hotels in the world (as awarded earlier by Conde Nast Traveller Magazine). In reality, it is a 5 star deluxe property (the alleged seven star status is not often corrected in the media, though).
The five-star hotels is the quintessential luxury hotel, offering thrills above and beyond the actual needs of the travel. They have restaurants and night spots that are world class, with food and entertainment that draw non-guests to sample it too.
Five-star hotels also tend to have opulent and expensive decorations; fancy gyms, swimming pools and spas. Major five-star chains compete to offer the most ludicrous thrills imaginable: Loews offers dog-walking services, while Conrad will let you order from a menu of pillows. Needless to say, all this comes at a steep price, and you're unlikely to be able to justify the expense of a five-star for ordinary business travel. The other downside to five-stardom is that hotels thatcan jump through all the hoops to achieve the rating are likely to be large and impersonal.
Major chains: Orient-Express Hotels, Conrad (Hilton), Pan Pacific Hotels and Resorts, St. Regis, Le Meridien and W (Starwood), InterContinental (IHG), JW Marriott and Ritz Carlton (Marriott), Fairmont, Raffles and Swissotel (FRHI), Shangri-La, Kempinski, Mandarin Oriental, Sofitel (Accor), Four Seasons, Regent, Langham International, Mövenpick Hotel and Resorts
The four-star hotel is a good business hotel. Everything works smoothly, there's Internet in every room, a well-equipped business center, they'll arrange your airport transfer and room service is palatable and only somewhat expensive. And your boss will probably not faint when they see the bill.
Major chains: Hilton, Marriott, Novotel (Accor), Crowne Plaza (IHG), Kimpton Hotels and Radisson BLU Hotels (Carlson Rezidor).
Three-star hotels are solid but dull. Your room will have an attached bathroom and there's probably a restaurant downstairs and 24-hour reception service.
Major chains: Mercure (Accor), Courtyard by Marriott, Holiday Inn (IHG), Best Western, Cyprus Hotels
Two stars means no-frills hotel. In many countries, two stars means that your room may have its own bathroom and there's almost certainly a TV and perhaps a telephone in your room, but rooms are bare-bones and you're unlikely to want to spend any more time than strictly necessary inside.
Major chains: Ibis (Accor), Comfort Inn, Motel 6, Super 8 and Etap
You don't see many of these, and with reason. One-stars are not just no-frills, but often downright dodgy: rooms are barely functional, shared bathrooms are somewhere down a corridor and the painted ladies from the all-hours karaoke bar next door dance the horizontal tango all night long in the room next to yours.