Segments let you group contacts for future reference or create lists of subsets of contacts.
When you import contacts (Contact List in the left sidebar > Import Contacts), you can choose a segment to add them to based on the specific attributes of that list.
From the Contact List you can filter your contacts and create a new segment by clicking Add to Segment.
You can create:
A Static Segment for contacts who match your filter right now
A Live Segment that will continuously update as contacts enter or exit the filters you set