What is the difference between adding elements to a list using the append method and concatenating lists?

The append method (for example, in Python — append, in Java — add for lists) adds a single element to the end of an existing list, modifying it in place.

List concatenation creates a new list by merging the elements of two lists without modifying the original ones.

Example in Java with ArrayList:

import java.util.ArrayList;

ArrayList<String> list1 = new ArrayList<>();
list1.add("a");

ArrayList<String> list2 = new ArrayList<>();
list2.add("b");

// append (add) an element to list1
list1.add("c"); // list1 now ["a", "c"]

// concatenate lists
ArrayList<String> combined = new ArrayList<>(list1);
combined.addAll(list2); // combined ["a", "c", "b"]

Thus, append adds a single element, while concatenation merges lists.