Solid regions have internal ultrasound echoes and are classified as echo poor, hypoechoic or hypoechogenic if there are few internal echoes. Hypoechoic structures appear dark in ultrasound imaging, more homogeneous structures are darker than heterogeneous.
Soft atherosclerotic plaque, liver adenoma or FNH appear with a nodular hypoechogenicity. As metastases close the blood vessels they infiltrate, tumor tissues become hypoechogenic after injection of contrast agent. Muscle appears relatively hypoechoic to tendon fibers, also articular hyaline cartilage appears hypoechoic.

Anatomic structures respond with characteristic features on ultrasound scanning.
There are some ultrasound terms, referring to the echo appearance, that describes tissue appearance in a uniform manner:

Tendons characteristically are hyperechoic on ultrasound because of the fibrillar pattern. Ligaments appear hyperechoic when the beam is perpendicular to the tissue. Peripheral nerves are hyperechoic relative to muscle.
Muscle appears relatively hypoechoic to tendon fibers. Close observation reveals hypoechoic muscle fibers separated by hyperechoic septae that converge on a hyperechoic aponeurosis. Articular hyaline cartilage appears hypoechoic. The presence of fluid within the joint outlining the cartilage produces a thin bright echo at this interface.
Sound beams do not penetrate the bone cortex. The very bright echo produced at the interface allows both recognition of the bone cortex but also can demonstrate fracture, spurring and bone callus bridging. Abnormal soft tissue calcification and ossification also produces bright reflective echoes.
Cysts or fluid filled areas are without internal echoes and are called echo free or anechoic and may demonstrate enhanced soft tissue echoes posterior to the fluid collection. Inflamed metatarsal bursae and calcaneal bursae clearly depict fluid swelling.

See also Beam Pattern and Zero Offset.

Anisotropic is an area or structure with varying acoustic properties according to the direction in which they are imaged. For example, the characteristic ultrasound appearances of tendons are hyperechoic, but they appear hypoechoic when the sound beam is not at right angles to the tendon fibers.