Swarm is an app developed by Foursquare that lets you comfortably and easily make plans with your friends. This app lets you know who's nearby and, even better, if they can or want to get together.
The most interesting feature of Swarm is that it lets you quickly indicate what you're planning (going out to eat at a restaurant, going out for drinks at a bar, heading to a club...), so any friend can see it and join if they want.
Even though you can write comments and communicate with other users directly from Swarm's interface, it also lets you quickly share your status on Twitter or other social networks.
Swarm's other possibilities include sharing photos in a way that is rather similar to that of Foursquare: you just check into a place and attach a photo to prove that you've been there.
Swarm is an interesting social app that lets you easily make plans with your friends. However, you have to have a lot of friends for Swarm to be really useful.