Just above the far right column on the lead list is a dropdown menuwhich allows you to apply processes to the list of leads which are displayed in your View. The options displayed in this dropdown vary based on User Permissions, but can include:
Sending a Mass Email
Assign Remove Type
Selection of the Leads is done with views, or by clicking on the checkbox at the far right of the Lead information as displayed in this screen.
If using the checkmark selection method, upon checking a lead, the name of the dropdown changes to "Apply to Checked" and any selection made from the dropdown will be applied to those leads alone.