Click on 'Contacts' and then 'Search'

Here you can search based on first name, last name, email, etc... 

You can also search tags.  You are able to include certain tags and exclude certain tags.  Example - You want to send a specific donation email to a list of specific donors.  You can do a search for everyone within the tag 'Donor'  but you want to exclude to people under the tag 'Donated Under $100'.  Now you just need to tag the search results.