Verb structures that include a continuous form are used when there is a requirement to focus on the duration of an action. The tenses that have this Core Characteristic are: 

Present Continuous

Future Continuous

Past Continuous

Present Perfect Continuous

Future Perfect Continuous

Past Perfect Continuous

While each of these tenses is covered in its separate section, it is useful to examine the similarities between the forms in order to understand the patterns that connect them. All these verb forms contain a Continuous Aspect. This is used to talk about temporary actions - actions that started at some point and will inevitably end - and to focus on the time period when this action was happening. The Continuous Aspect is useful because it allows the speaker to talk about parallel occurrences and to show which actions were quick and which actions were enduring:

In this sentence, the Past Continuous verb "I was working out" shows that this action happened over a longer period, while the past simple verb "when you called" was a short action.