Custom segments enable you to save user segment queries and re-target such user segments with more campaigns without having to apply same filters on user attributes/events and creating the segment again.
Creating Custom Segments
You can create custom segments by saving the segment while creating a segmentation query by clicking on the below button. Read more about creating user segments here.
You can also save a custom segment from Recent Queries section by clicking on the Save button in a query. Read more about Recent Queries section here.
Using Custom Segments
Custom segments are useful while creating campaigns. You can pick the custom segment from the Target Audience drop down.
With custom segments you also have an option to exclude users from specific custom segments by selecting from this drop down while creating another segment.