Hemorrhoids are swollen veins and blood vessels located in your anus.
When those veins and blood vessels in your anus swell up they become painful and that's what is called a hemorrhoid.
Anal fissures are small tears in the skin around your anus and they can seem similar but Anal Fissures and Hemorrhoids are different although in some cases Anal Fissures can be a cause of Hemorrhoids although Hemorrhoids are caused by other things.
Hemorrhoids can be caused by pressure in the lower rectum that is increased when you strain during a bowel movement or sitting for too long on the toilet causing pressure to increase in your rectum.