A segment works like a filter in a list and permit to select the better target for your message.
The requirements can be geographic, professional, behavioral…
A segment is a dynamic element. So, if you add some new subscribers to your list, the segment will be automatically updated.
A segment can use default fields update langs set en=email address, firstname, lastname…), custom fields and actions update langs set en=opening, clicks, status…)
All these requirements can be uses as conditions.