The key difference between a linear array and a phased array is the way in which the transducer is fired.
In a linear array, the transducer elements are fired in small subgroups along the length of the array. A phased array is fired differently.
There are two subcategories of linear arrays:
rectangular linear arrays
convex (curved) linear arrays
Linear arrays are the most commonly used array transducers used in diagnostic ultrasound.
Due to their wide field of view, convex linear arrays are used for abdominal, and obstetrical and gynecologic scanning.
Rectangular linear arrays are used for scanning superficial structures such as thyroid, breast, and scrotum. They are also used to scan carotids, leg veins and other superficial vessels.
Please watch my video on linear array transducers to learn more detail: