What is difference between a follicle on ovary and a cyst on ovary?

Egg vs Fluid: A follicle is a normal part of the ovary that holds the egg. Each month a follicle is triggered by hormones to mature so that the egg can be ovulated. Cysts are fluid filled and larger than a normal follicle. There are many types of cysts. Most are harmless but some could be associated with different diseases, ex polycystic ovary syndrome or endometriosis.

None: A follicle is a cyst in the ovary. However ultrasound cannot differentiate "functional" cysts (supposed to be there) from a neoplastic one (benign or pre-malignant, or malignant one). We use ultrasound criteria and risk factors to decide if anything needs to be done, but usually a small, single, thin-walled cyst of the ovary is normal and is supposed to be there. Serial ultraounds are helpful.