Component markup resides in the .cmp file. Markup consists of HTML elements, aura components (aura:component provides default rendering implementation), and other custom child components. Custom components start with the default namespace c. Apart from the c namespace, Salesforce provides out-of-the-box components with a UI and the Lightning namespace (components with the Lightning namespace are referred to as base components).
It is recommended to use a new Lightning namespace when using out-of-the-box components because Lightning namespace components automatically apply the Salesforce Lightning Design System (SLDS), CSS, and also provide in-built JavaScript functions such as validations, currency formatting, and localization.
The syntax for the component is as follows:
<aura:component>
<!-- Optional coponent attributes here -->
<!-- Optional HTML markup -->
</aura:component>