External haemorrhoids originate below the dentate line. They are bulging around the anal verge, very painful when inflamed, sometimes itchy.

Internal hemorrhoids originate above the dentate line. They often cause painless bleeding and sometimes come out of anal orifice during defecation.

No, you do not always need surgery. Mild hemorrhoids are often managed with Ayurvedic medicines and Ksharakarma. More advanced cases may need a procedure such as Ksharasutra ligation, depending on your symptoms and grade of piles.

Strangulation of hemorrhoidal mass causes ischemic necrosis. The kshara in the thread causes sclerosis of local tissue.

No such guarantee, but the incidence of recurrence is less as it cuts the swollen mass as well as scleroses the local tissue.