What are handles in magento (layout)?

Handles are basically used for controlling the structure of the page like which block will be displayed and where.
First level child elements of the node are called layout handles. Every page request can have several unique Handles. The handle is called for every page. handle for products belongs to virtual product type, PRODUCT_TYPE_simple is called for product details page of simple product type and PRODUCT_TYPE_virtual is called for the virtual product detail page and customer_logged_in handle is called only if customer is logged in.

The muster_index_index handle is created by combining the frontName (muster), Action Controller (index), and Action Controller Action Method (index) into a single string and this handle will be called only when /muster/index/index url is accessed.

<layout version="0.1.0">
<muster_index_index>
<reference name="root">
<block type="page/html" name="root" output="toHtml" template="../somepage.phtml" />
</reference>
</muster_index_index>
</layout>