Component:

<!--TabComponent-->
<aura:component>
    <aura:attribute name="selTabId" type="String" default="tab1" />
    <div aura:id="data-entry">
        <lightning:tabset onselect="{!c.tabSelected}" variant="default" selectedTabId="{!v.selTabId}" >
            <lightning:tab label="Mobile" aura:id="tab1" tabindex="1" id="tab1" title="Mobile">
                <p>Apple</p>
                <p>Samsung</p>
                <p>Lenovo</p>
            </lightning:tab>
            
            <lightning:tab label="Laptop" aura:id="tab2" tabindex="2" id="tab2" title="Laptop">
                <p>Apple</p>
                <p>Lenovo</p>
                <p>Dell</p>
            </lightning:tab>      
        </lightning:tabset>
    </div>
</aura:component>

Component Controller:

({
    tabSelected: function(component,event,helper) {
        alert(component.get("v.selTabId"));
    }
})

Output: