The two main parts of a template are the content management code and the theme. These should be kept separate so that the theme can easily be changed without affecting the structure. For example, the styles should be defined in an external style sheet and not embedded in the HTML code of the pages. The structure of a website will be defined by a set of templates. It is good practise to make the templates modular rather than having page templates containing all the code for each page. This allows site-wide changes to be easily applied. For example, the footer may be mostly identical between pages.
While people having skills in HTML and other web scripting languages can opt for designing their own site it is not a viable choice for everyone. Some of the web designers use pre defined and readymade web templates for quick and effective site designing.