$Browser in Lightning Component:

The $Browser global value provider returns information about the hardware and operating system of the browser accessing the application.

<aura:component>

    {!$Browser.isTablet}

    {!$Browser.isPhone}

    {!$Browser.isAndroid}

    {!$Browser.formFactor}

</aura:component>

Browser.App

<aura:application extends="force:slds">

    <c:BrowserComponent/>

</aura:application>

I am accessing application through desktop.

Output:

falsefalsefalseDESKTOP

LINK:

https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/expr_browser_value_provider.htm