What is session in Rails?

The Rails session is a server-side (or cookie-based) key-value store that persists data across requests for a single user. By default, Rails uses cookie-based sessions — encrypted and signed session data stored in a browser cookie (CookieStore). Set: session[:user_id] = user.id. Read: session[:user_id]. Clear: session.delete(:user_id) or reset_session. Cookie sessions are limited to ~4KB. For larger or more secure sessions, use ActiveRecord::SessionStore or Redis-backed sessions.