Contractor

Dont be put off by the term contractor. Your builder will most likely be your contractor. The most important use of teh word contractor should be in your contract with your builder. It is the clearest way to define the hierarchy that will be necessary for your project.
Two reasons why this is important are that one, it will help to define the lines of communication and help avoid confusion, whic in turn inevitably leads to financial issues, and secondly, who pays who.
As the client, you are at the top of the chain, and ultimately the most responsible. In order to get the job done, you therefore employ suitable proffesionals to do the work for you, and you pay them for this!

The Contractor or builder, will be employed by you to build your extension - he may well employ sub-contractors who specialise in trades that your contractor doesnt, he is responsible to employ the sub-contractors under the terms he wishes, in respect of payment etc, but certain parts must comply with the contract he is working under with you, such as hours of work.
Depending on the type of contract and project, there may be 'specialist' or 'nominated' contractors who you wish to carry out bvery specific or specialised parts of the job. Such trades may be railings, or kitchen installers.