The General Data Protection Regulation (GDPR) strengthens the obligation of information and transparency with regards to data subjects whose data you process. The information required is defined by Articles 12, 13 and 14 of the GDPR.
Whenever you plan to collect personal data, and regardless of the medium used, you must provide a set of mandatory information to the individuals concerned before the collection of their data.
Generally, these notices are referred to as privacy policies, privacy notices or data protection notices. The data subjects can be your customers, prospects, website users, suppliers, etc.
The details of transparency guidelines (Article 29 Data Protection Working Party) can be found in this document.