Hi Suzanne

With CRM you can create an number of custom notifications / email alerts. For your example you might want to look at creating a 'Quick' Notification. For Standard entities this can be done by going to Administration | Customisation | Lead within this area at the top is a tab marked 'Notification' 

In here you can configure CRM to use either the on screen notification or send an email when a criteria is met (e.g. when the Assigned To field becomes populated)

For more advanced criteria, you might want to think about editing the Lead Workflow 

Building a workflow (sagecrm.com)