WHAT IS THEME ?
A WordPress theme changes the design of your website, often including its layout. Changing your theme changes how your site looks on the front-end, i.e. what a visitor sees when they browse to your site on the web. There are thousands of free WordPress themes in the WordPress.org Theme Directory, though many WordPress sites use custom themes.
Your template is the layout or blueprint of your site; it helps structure your website, giving you areas to place pictures and text, or things like navigation bars and other widgets. Your template should vary depending on the kind of page you are creating, but will share similar themes and patterns across your site. A layout for a gallery page will differ to the layout for your blog page, which in turn, will differ from your contact page; however, they will also have things in common, for example, the location of your navigation bar.