A trade mark is a word, phrase or visual symbol which you use to distinguish your goods and services from the competition. Trade marks are often described by the TM symbol. In the UK, you are allowed to use any trade mark you like provided that it is not prohibited or registered for a competing brand. However, you only enjoy protection for your trade mark if it is a Registered trade mark. Registration is through the Intellectual Property Office and costs £160 or £200 for a single mark, subject to whether you want pre-advice on the mark.
Normally, you would register your logo as a trade mark, though you can register a distinctive phrase as well.
Generally speaking, the more distinctive the name of your mark, the less distinctive the visual identity needs to be for it to be registerable. If the name is fairly generic, it will need a more distinctive visual shape.
Trade marks are registered in categories known as Classes. This enables non-competing goods or services to have a similar name or look.
You cannot register a trade mark which describes your goods or services. For example, 'Wholemeal bread' is a description, and cannot be registered for a food product. On the other hand, if you were an IT company and wanted to call yourself 'Wholemeal', then this would be registerable.
When choosing a brand name, it is important to check whether it has previously been registered by someone else. The Intellectual Property Office's database can be searched for free here.
Note that owning a web-address or URL does not give you the right to use a trade mark.