Based on information gathered from their activity (e.g. date they were added to your list, number of times they opened a campaign, number of times they clicked on a link, etc.) you can design your segments and filter them as you see fit.
You can even take things further by setting multiple segmentation criteria that your audience has to fulfill, and narrow down the list of recipients contained inside your segment. This way you can send fully targeted campaigns to the people you deem most likely to respond positively to one of your campaigns.
During the list segmentation procedure make sure you do the following:
1. Specify whether your segment needs to fulfill All or Any of the criteria you set.
2. Select to Add More Criteria to further refine the conditions of your segment.
If you feel you need to learn more about segmentation before creating your own list segments, it's worth looking at a concrete example on how you can create a segment out of recipients who haven't opened a specific email campaign that you've sent out.