Parsley, unlike
chives, for example, can be used both raw and cooked, but when cooking, don't neglect the stems, also known as "tails".
It's not obvious at first glance, but much of parsley's flavour is in the stems, so it's a good idea to put them in your saucepan for cooking.

Let's take a recipe for
mussels marinière as an example: heat a base of dry white wine with onion and parsley, and open the mussels (I'm oversimplifying).
Well, the big difference here is that the recipe uses the whole parsley, not just the leaves, because parsley tails are just as, if not more, important than the leaves when cooking. You can't eat them, but they add a lot to the taste, so you have to read "whole parsley sprigs (leaves + stems)" in the recipe, even if it says "parsley" in the list of ingredients, because it's very important for the recipe.
As they can't be eaten, it's a good idea to tie them with string into small bunches for easy removal at the end of cooking.
As a general rule, when a recipe calls for whole parsley, we often indicate "parsley sprig" instead of "parsley".
If you ever come across an old cookery book (before 1960/50, roughly speaking), you'll see that it's not uncommon to find recipes with "parsley tails" as an ingredient, mainly sauces in fact.
And parsley is one of the essential ingredients in the famous
bouquet garni I've already told you about.
To sum up: for a recipe using cooked parsley, it's essential to use the whole sprig, leaves + stems.