A user account grants a person access to the data in a Google account with a web browser. User accounts are for people connecting with a web browser. The service account gives applications remote access to the data in a Google account.
Google Service Accounts
A service account is a special kind of Google account and authentication that is used by programs such as ETL+ rather than a person. ETL+ uses a service account to authorize access to your Google Sheets. The service account's roles control which Google resources ETL+ can access.
A service account is identified by its email address, which is unique to the account.
Service accounts use public/private RSA key pairs rather than passwords. The RSA key required to access a service account is created by Google and saved in a
Differences between a service account and a user account
Service accounts differ from user accounts in a few key ways:
Service accounts do not have passwords, and cannot log in via browsers or cookies.
Service accounts are associated with public/private RSA key pairs that are used for authentication.
For more see https://cloud.google.com/iam/docs/service-accounts