What is it?
The basic difference between spotting and a period is the quantity of blood discharged through the vagina. During your period there is usually a heavy flow of blood that can last for an average of 3-5 days. With spotting, the discharge of blood is sporadic or irregular.
Spotting may occur as part of the menstrual cycle, however it can be an indication of pregnancy or an underlying condition so please see a doctor if it occurs.
Pay attention to what colour the blood is (and how heavy it is) as this may be helpful if you need to explain to a doctor. Bright red is usually fresh blood, pink is diluted, brown is days-old blood, and black is very old blood.