Also called trip planner; simply put, it's an app that allows you to have all your travel plans in a single place, usually ordered in a chronological way. Kind of like the folder you would carry around with your printed flight and hotel reservations...only there's no need to print now. Plus, they usually offer additional features that your folder can't have, no matter how fancy it is(and if it does, call us. We want to know about it), like checking available seats or tell you when you have to leave to the airport.
There are two basic "models" an app of this kind can follow:
a) You add reservations that are then compiled into an itinerary
b) You first create a trip where you later add the reservations / items / plans
1. Add reservations (flight, hotel, events...)
2. Reservations are compiled into an itinerary
1. Create a trip
2. Add reservations, items, plans...
In the end, though, the final result is always an itinerary: an organised list of your travel plans with its key information.
When you add a flight, you also get regular updates on the status of your flight and things like delays or cancellations. Some apps go the extra mile and offer you other types of reminders, like alternate flights (TripIt) or traffic reports to the airport (Sam).
This means all the apps mentioned below have a similar structure, or at least some screens that have the same objective. For instance, you usually start with an empty screen. You then either create a trip or directly add a reservation (that is categorised inside a trip). The number of taps needed until you can see the specific details of your reservation (like your departure time or a hotel direction) will vary depending on the app, with some of them trading accessibility for a less cluttered interface.
All the apps from the list allow you to add different types of reservations (called plans or items in some cases) except for one, App in the air, which focuses on air travel. Still, it's similar (and complete) enough that we thought it should be included.