An observable and the operators you use on it will execute and notify observers on the same thread on which the subscribe operator is being used.
Schedulers are an abstraction that let you specify distinct queues on which to perform work. It is always a good idea to move all the intensive work off the main thread to keep it as responsive as possible for users, and schedulers make it easy to do this. They can be Serial or Concurrent.
You will use a serial scheduler if you want to ensure that work is carried out on that queue in the order it was added to the queue, otherwise you will typically just use a concurrent queue.
There are built-in scheduler types that work with Grand Central Dispatch (GCD) queues and NSOperationQueues
