Delayed onset muscle soreness
DOMS is the ache that turns up a day or two after unfamiliar training — new exercise, more weight, or the return after time off. It peaks somewhere between 24 and 72 hours, and it is caused by microscopic damage to muscle fibres as they adapt.
It is a normal training response, not an injury. It is also a poor measure of how good a session was — the absence of soreness does not mean the session was wasted.
How to tell it from an injury: DOMS is spread across a whole muscle group, affects both sides roughly equally, and eases as you move. An injury is usually one specific point, on one side, and gets worse with use.
Massage will not make DOMS vanish, but it genuinely eases the stiffness and gets you moving comfortably sooner.
Cramp
Sudden, involuntary and unmistakable. Most often the calf or hamstring, often during or after hard effort, and often at night.
Contributing factors are usually some combination of fatigue, dehydration, electrolyte loss, and a muscle that was already tight before you started. That last one is where treatment helps — muscles carrying trigger points and chronic tightness cramp far more readily than healthy ones.
Frequent cramping without an obvious cause is worth mentioning to your GP. It is occasionally a sign of something needing investigation, particularly if it is new, worsening, or accompanied by weakness.
What treatment involves
Regular soft tissue work as maintenance rather than repair — keeping tissue quality good so that it recovers faster and cramps less. This is the most common reason people book in regularly rather than only when something breaks.