The progressive aspect expresses the dynamic quality of actions that are in progress.
The progressive aspect expresses the dynamic quality of actions that are in progress while the continuous aspect expresses the state of the subject that is continuing the action. For instance, "Tom is reading" can express dynamic activity: "Tom is reading a book" - i.e. right now (progressive aspect), or Tom's current state: "Tom is reading for a degree" - i.e. Tom is a student (continuous aspect). The aspect can often be ambiguous; "Tom is reading Ulysses" may describe his current activity (it's in his hand), or the state of having started, but not yet finished, the book (it's in his bag).