1. The final chord is a tonic chord—a chord that is built on the tonic, or first, scale degree of a major or minor scale.
2. The penultimate chord (the chord immediately preceding the final chord) is a non-tonic chord, often the dominant chord (the chord built on the fifth scale degree) or the subdominant chord (the chord built on the fourth scale degree).
Therefore, what all imperfect cadences have in common is that they end with a tonic chord, but they are preceded by a different chord, typically the dominant or subdominant.