Update components.md

Author: hootlex

src/guide/components.md

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 ##What are Components?
-Components are one of the most powerful features of Vue.js, helping you extend basic HTML elements and create reusable code.
-At a high level, Components are markers on a DOM element that tell Vue.js compiler to attach a specified behavior to that DOM element, or even to transform the DOM element and its children.
+Components are one of the most powerful features of Vue.js, helping you extend basic HTML elements and create reusable code. At a high level, Components are custom elements that Vue.js’ compiler attaches a specified behavior, or even transforms the elements and their children.
+
+In some cases, a Component is indicated with a special attribute on an HTML element.
 
 ## Using Components