Apps are basically links to webpages. The app adds a tile to your menu on the chromebook and when you click on it, your web browser opens and takes you to that website. If you download an app from the google webstore and don't like it, simply right-click the tile on your menu of apps and you can remove the app from chrome.
Extensions are things that are added to your browser. When you add an extension, it adds something to your browser. This is usually some type of functionality. In previous posts I have talked about 2 extensions, Skiblz cam and speed dial. These are things that allow me to do different things in my browser. Speed dial allows me to change the look of my new tab to a set of tiles. Skiblz cam allows me to record a screencast of what I am doing in the browser. These are embedded in your browser. To remove an extension, you must click on the chrome browser menu in the upper right hand corner of the browser,