A circular arc or simply arc is a part of a circle or the curve between any two points on a circle. In the image below, a circular arc is shown in orange.
Whenever you use a compass and you draw something that is not a circle, you are drawing an arc. An arc can be a minor arc, a major arc, or a semicircle.
Looking at the circle below, the minor arc is shown in orange and the major arc is shown in blue.
In the circle below, you can see two semicircles, one in orange and another one in blue.
Notice that a minor arc is smaller than a semicircle and a major arc is bigger than a semicircle.
A minor arc is an arc whose measure is less than 180 degrees.
A major arc is an arc whose measure is bigger than 180 degrees.
A semicircle is an arc whose measure is equal to 180 degrees.