Segments let you group contacts for future reference or create lists of subsets of contacts.
When you import contacts (Import Contacts in the left sidebar), you can choose a segment to add them to based on the specific attributes of that list.
From the Contact List you can apply relevant filters and create a new segment by clicking Segment on the top right of the page.
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:
Find out more using segments: How do I manage my segments?