Qalqalah can be found in five letters. They are combined in the phrase قطب جد. These five letters all have the attribute of Jahr (preventing the airflow when pronouncing them) and Shiddah (preventing the flow of sound when pronouncing them).
Qalqalah is essentially opening up the point of articulation of the letter after pronouncing the letter with sukun in order to let a burst of airflow out and a slight sound.
Qalqalah is only observed on these letters when they have a sukun. When they have a vowel sign, they are pronounced normally without any qalqalah.
These examples are all from Suratul Masad (Chapter 111).
1. The Strongest - If the Qalqalah letter comes at the end of the word or verse, has a shaddah, and we stop on it.
Example is the last ب in the ayah - تبت يدا أبي لهب وتبّ
2. The Intermediate - If the Qalqalah letter comes at the end of the word or verse, does not have a shaddah, and we stop on it.
Example is the last ب in the ayah - ما أغنى عنه ماله وما كسب
3. The Lightest - If the Qalqalah letter comes in the middle of a word.
Example is the ب in the word حبل in this ayah - في جيدها حبل من مسد