Make use of wrapping layouts in Craft
Craft's uses the Twig templating language. The official Twig documentation http://twig.sensiolabs.org is really good, highly recommended to read most of it!

For your specific problem have a look at the section about the extends tag. It explains how to define blocks and extend other template files. Another important technique is including partials into a template with the include tag.

For a good and rather simple example I would recommend to read the source code of Pixel and Tonic's On The Rocks Craft demo page.