Quite often, in the literature and documentation terms, Hot and Cold Observables can be found.

Cold Observables are the most common Observable type. For example, one can be created with the following code:

Observable.just("First item", "Second item")
        .subscribe();

Cold Observable means that the items won't be emitted by the Observable until there is a Subscriber. This means that before the .subscribe() element is called, no items will be produced and thus none of the items that are intended to be omitted will be missed; everything will be processed.

A Hot Observable is an Observable that will begin producing (emitting) items internally as soon as it is created. For example, in the later chapters we will see that Twitter can be a Hot Observable--the status updates there are produced constantly and it doesn't matter if there is something that is ready to receive them (such as Subscription). If there were no subscriptions to the Observable, it means that the updates will be lost.