What are the differences between a muscle sprain and a strain?

Joints can sprain: A muscle "strain" is when muscle fibers get stretched too far and/or some fibers tear. People generally call it a tear if enough fibers break, leading to some bleeding and swelling inside the muscle, resolving later with some healed scar tissue and some tightness in the area (less elastic). A joint sprains when its ligaments (whitish fibers) are stretched too far and break. Muscles don't "sprain".

See below: Muscle fibers are injured that causes muscle strain. Ligaments can be injured and the terminology is ligament sprain.