What is a network?

If you walk down into the Kendall Square subway station in Cambridge, Massachusetts, through the turnstiles and past the Paul Matisse art installation, you will see a common feature of all subways: a map. 

If you have ever used a subway map, you know that they are not quite like other maps, which tend to be concerned with details like distance and terrain. Subway maps leave all that out, showing you the bare minimum you need to know to get from one place to another: which stations are connected to each other.

When you're using a subway, it doesn't matter how long a stretch of track is, what exact direction it goes in, or even whether it is above ground, underground, or under water. All you have to do is get on at one station and get off at another, possibly with some transfers between the two. Such a map is an excellent example of the types of networks used by network scientists.