What is the difference between a Profile and a Permission Set?

A Profile is the primary security baseline assigned to every user (one per user). It controls object permissions, field-level security, app visibility, login hours, and IP ranges. A Permission Set extends a user's access beyond their profile without changing it — a user can hold multiple permission sets. Permission Set Groups bundle related permission sets together. Salesforce's long-term direction is to shift all permissions away from profiles and into permission sets, making profiles contain only a minimum baseline.