What is <view/> XML tag in layouts

Actually both are same. In the first xml it says that it would be a view of type android.support.design.widget.Snackbar$SnackbarLayout (defined in the class property)

  <view xmlns:android="http://schemas.android.com/apk/res/android"  class="android.support.design.widget.Snackbar$SnackbarLayout"

In second one it's directly declaring using the custom class.

Second format could be used only if the Custom view is not defined as an inner class

From Android Documentation

We now have our custom component, but how can we use it? In the NotePad example, the custom component is used directly from the declarative layout, so take a look at note_editor.xml in the res/layout folder.

<view  class="com.android.notepad.NoteEditor$MyEditText"   id="@+id/note"  android:layout_width="fill_parent"  android:layout_height="fill_parent"  android:background="@android:drawable/empty"  android:padding="10dip"  android:scrollbars="vertical"  android:fadingEdge="vertical" />

The custom component is created as a generic view in the XML, and the class is specified using the full package. Note also that the inner class we defined is referenced using the NoteEditor$MyEditText notation which is a standard way to refer to inner classes in the Java programming language.

If your custom View component is not defined as an inner class, then you can, alternatively, declare the View component with the XML element name, and exclude the class attribute. For example:

    <com.android.notepad.MyEditText      id="@+id/note"      ... />

Notice that the MyEditText class is now a separate class file. When the class is nested in the NoteEditor class, this technique will not work.

The other attributes and parameters in the definition are the ones passed into the custom component constructor, and then passed through to the EditText constructor, so they are the same parameters that you would use for an EditText view. Note that it is possible to add your own parameters as well, and we will touch on this again below.

And that's all there is to it. Admittedly this is a simple case, but that's the point — creating custom components is only as complicated as you need it to be.

A more sophisticated component may override even more on... methods and introduce some of its own helper methods, substantially customizing its properties and behavior. The only limit is your imagination and what you need the component to do.