Colonic diverticulosis is best diagnosed by
"Diverticula are best seen on barium enema. Colonoscopy is a less sensitive means of detecting diverticula." - CMDT 2007

Diverticulosis refers to the presence of multiple diverticula
Diverticulitis refers to inflammation and infection associated with a diverticulum.

Best investigation for
Diverticulosis - Barium enema
Diverticulitis - CT scan

On barium, enema diverticulosis shows characteristic 'sawtooth' appearance
Colonoscopy
Nuclear scan
Barium enema
CT scan