Farming texts and MAFF give slightly different definitions, but generally:

Boar - Uncastrated male pig, whether used for meat or breeding.
Sow - Adult female pig used for breeding, generally called this from her second pregnancy.
Gilt - Young female pig between weaning age and time of her first litter. The term is also applied to female pigs kept for meat.
Piglet - Young pig, usually from birth until weaning.
Weaner - Pig from weaning age (usually 21 to 24 days) to the age of 10 weeks
Rearing pig - Pigs from 10 weeks to slaughter