We consider a cadence to be weak or strong depending on the strength of the sense of resolution it creates.
Authentic cadences are considered to be the strongest. Perfect authentic cadences are stronger than imperfect ones. Because of their strength, perfect authentic cadences naturally occur at the end of compositions. However, not every end of a composition contains a perfect authentic cadence. Sometimes, a composer deliberately makes an ending that sounds open-ended.
A composition or movement that ends with an authentic cadence in the same key in which it begins is harmonically closed. Most of compositions and movements are harmonically closed. The same term can be also used with smaller parts of compositions.
Authentic cadences have the ability to confirm the key in which they occur. This is important when modulating to a new key and will be explained further in chapter Modulations.
Deceptive cadences are considered the weakest.
Authentic and plagal cadences progress to the tonic chord, which is why they are considered stronger than half cadences and deceptive cadences, which do not progress to the tonic chord.
Thus, we can order cadences according to their strength, from the strongest to the weakest:
Perfect authentic cadences
Imperfect authentic cadences
Plagal cadences
Half cadences
Deceptive cadences.