lamb, hogget, and mutton, generically sheep meat, are the meat of domestic sheep, ovis aries.
a sheep in its first year is a lamb and its meat is also lamb. a sheep in its second year and its meat are hogget. older sheep meat is mutton. generally, "hogget" and "sheep meat" are not used by consumers outside new zealand and australia, only the industry.
in south asian and caribbean cuisine, "mutton" often means goat meat. at various times and places, "mutton" or "goat mutton" has occasionally been used to mean goat meat.
lamb is the most expensive of the three types and in recent decades sheep meat is increasingly only retailed as "lamb", sometimes stretching the accepted distinctions given above. the stronger-tasting mutton is now hard to find in many areas, despite the efforts of the mutton renaissance campaign in the uk. in australia, the term prime lamb is often used to refer to lambs raised for meat. other languages, for example french, spanish, italian and arabic, make similar or even more detailed, distinctions among sheep meats by age and sometimes by sex and diet, though these languages do not always use different words to refer to the animal and its meat — for example, lechazo in spanish refers to meat from milk-fed (unweaned) lambs.