What is a gland?
A gland refers to a type of tissue. (1)
A gland is a unit of cells that work together to create and secrete substances. (4)

What are types of glands?
adrenal glands
endocrine glands
exocrine glands

What are types of glands, based on the products they produce?
serous glands (3)
mucous glands (3)
seromucous glands (3)

1. https://my.clevelandclinic.org/health/articles/22464-hormones
2. https://www.healthline.com/health/the-endocrine-system#function
3. https://open.oregonstate.education/aandp/chapter/4-2-epithelial-tissue/
4. https://my.clevelandclinic.org/health/body/22947-exocrine-glands