How can I create a segment within a campaign/trigger?

Campaigns and triggers, at the highest level, can be configured to send to specific segment(s).

Within a campaign/trigger, each step can be customized as well to send to specific subgroups of that segment. To configure this, check the Conditionally send? box under the desired step. Here, you can add additional filters to only send that step to a sub-set of the segment.

In this example, this campaign is targeted for All Active Students, but Step 1 will only send if the client goes to "Smithtown" school.