Descender is a part of the letter which extends below the baseline, in the opposite way from ascender. Most of the descenders belong to lower-case characters, such as g, j, p, q and y. However, depending on the font, capital letters can also have descenders: usually we are speaking about Q and J. Some descenders are also called “tails” because of their curve: this works for letters j, y and Q, unlike the p and q where descenders are vertical. Letter g’s descender is often called “loop”. In some typefaces you can also notice the letter f with a descender. Many letters can slightly extend below the baseline but this is commonly known as overshoot, not to be mistaken for descender.
Descenders as elements have some varieties to be considered as well.