登记 means 'to register' in a general, often administrative sense — like registering at a hotel, signing in at an office, or recording personal details in an official system. It is broader than 报名 (to sign up for a specific event or course) and different from 注册 (to register an account or enroll in a system, especially online). You would 登记 a guest's name, but 报名 for a competition and 注册 for a website.