<-- You can tell if a location is an outpost by the outpost icon on your World Map (key "M").
An Outpost is smaller than a city, and does not offer the full set of infrastructure. Usually you'll only find a merchant, a Xunlai Agent, often a skill trainer and sometimes a Trader or Collector. Unlike mission locations (below), outposts will always offer something extra beyond the basic merchant and agent. It could be a trainer or a quest giving NPC or something else.
For a list of outposts, see Outpost Locations