Segments are dynamic groupings of contacts. Only contacts that match the Query will be included in the segment.
Your account may have already had some default segments saved in your contacts. The video tutorial shows you how to make a couple of simple segments and save them to your account.
But, the segmentation tool is very powerful and will allow you to build very complex queries based on a wide range of criteria.
Navigate to your contacts screen and under the Manage Contacts, choose Create segment
Input the rules of your preference:
The rules are divided into General, Statistics and Custom categories.
General - data that is contained within your contact
Statistics - filter your contacts by their performance
Custom - this is the category that contains your own custom fields
Clicking the "Evaluate" button will allow you to see how many contacts adhering to your chosen rules are present on your account.
When you are satisfied with the segment's setup, just hit Save.