Definitely bear in mind that a rating on a customer feedback and opinion site, such as TripAdvisor, is a rating of the traveller's view of their hotel stay and not a hotel grade.
According to this USA Today article, the hotel rating system was started by Mobil, the oil company, in the 1950s as they also produced the Mobil Travel Guides. They sent out inspectors to check more than 550 points of criteria to give travellers a trusted consistency in grading.
To reach a five star rating award, hotels had to meet an extra 20 detailed service benchmarks including fresh flowers in each room and 24-hour room service, standards which are still used today by hotels to judge themselves.
An international hotel ratings system would never really work as a country's standards can't usually be compared. A three star hotel in the UK can mean a chain hotel near a motorway with a family restaurant attached and a three star in Turkey could mean a beach hotel with poolside service.
Many countries have a government inspection system and the countries of the UK have linked up to try and have some uniformity.
While it's good to do some double-checking when you hear a hotel's star rating there are some universal guidelines. We all know a four or five star hotel is going to be better than a one or two star hotel. At four and five star there is some expectation of luxury and higher cost and at one or two star a small budget room is likely. But that doesn't mean a one star won't have all you need for a good night's sleep; the room just won't have all the nice extras.
This breakdown should give you some guidance when choosing a level of hotel.
1 Star: Budget, no-frills accommodation. Yes, there will be a bed and bed linen but there may be a shared bathroom down the corridor. Expect the hotel to be small and probably run by the owner. It will be simple but meet all basic needs.
2 Star: Good value, clean and comfortable but still basic. Limited amenities but could include a TV and phone in the room. There should be dining facilities on site but room service is unlikely.
3 Star: Different room sizes available and generally more rooms. A convenient location which may mean by the side of motorway. Rooms are more spacious and on-site restaurants offer breakfast, lunch and dinner. All rooms have a phone and there is a dedicated Receptionist in a dedicated Hotel Lobby. Hotels may well have extra facilities such as a Fitness Centre or conference rooms.
4 Star: Large hotels with good service in a great location. Rooms are more luxuriously furnished and room service is available. Staff should have received more training and there may well be a Concierge available. This is a superior property with more facilities.
5 Star: A high level of luxury and flawless personal service. These hotels have elegance and style throughout with quality linens and extra facilities in the room such as a music player, more TV channels on a larger TV and luxury toiletries. Room service is 24-hour and hotel staff may include a doorman and valet parking too. An evening turn-down service is often offered.