A trade mark is a sign which attracts purchasers to your product or services as opposed to competing products and services. In order to do so the sign needs to be distinctive. As a consequence it should not be descriptive of the product or services.
Consider well-known trade marks such as Cadbury for confectionery or Raleigh for bicycles. These are distinctive and not descriptive. Contrast those with the fictitious trade marks Yummychocs for confectionery or Betterbikes for bicycles.
A trade mark could be not only a word but also a sign. Consider the well-known trade mark of the Shell for oil. A trade mark could also comprise a smell, sound or colour or indeed any distinguishing feature that can be defined and reproduced. Need less to say, many trade marks are made up of a combination of one or more elements trade marks.
There are also special trade marks, known as certification marks and collective marks; the former requires authorisation by the owner and the later is basically a badge of membership. Consider the well-known trade marks ORGANIC SOIL ASSOCIATION and CORGI.
A trade mark can be registered or merely adopted without registration but there are significant advantages to a trade mark which is registered.