Let’s cover this in 2 parts.

G Suite is the name given to the overall suite of tools and apps you’ll have access to from your Google account. Included with your account you’ll have access to: Gmail, Google Calendar, Google Contacts, Hangouts or Meet, Google Drive including Docs, Sheets, Slides, Drawings and Forms, Google Groups and Google Sites. All of these apps sit within G Suite.

Gmail is the email app within G Suite. It’s the app you’ll be using to send, manage and organise your email.

Previously you may have used other applications such as Microsoft Outlook or Lotus Notes.