X-rays can be used to diagnose bone injuries, heart and lung disease, gastrointestinal problems, including swallowed foreign objects, as well as identify urinary system abnormalities and more.
Ultrasound focuses more on the diagnosis of soft tissue ailments, and often is used in conjunction with x-rays. For example in a pregnant dog, the x-rays can be used to determine how many pups are present, while the ultrasound can check fetal heartbeats.