Consistency helps people use your site effectively. It also helps to brand your site.
Make sure that important elements such as navigation and page headings/descriptions are in the same place on every page. Visitors should be able to know from the first page they see where those elements are, and be able to glance at the same areas on each page to establish where they have arrived in the site while browsing.
Make sure your logo is on every page in the same position. Somewhere near the top is usually the best place. Keep the colours and layout as consistent as possible across the site.
People should know immediately whether they are still on your site or have clicked an outside link by the style. Use the same colours and fonts for text on every page.