The main difference here is that a Privacy Policy is required by law if you collect or use any personal information from your users, e.g. email addresses, first and last names etc. while a Terms & Conditions agreement sets forth terms, conditions, requirements, and clauses relating to the use of your website or mobile/desktop app, e.g. copyright protection, account terminations in cases of abuses, and so on.
Depending on your website or mobile/desktop app, you'll need either a Privacy Policy agreement and a Terms and Conditions (T&C) agreement, or both.
Each of these two legal agreements serves different purposes for both you (the company operating the website/mobile app) and your users.
This article will break down the differences further so you know which to use and when.