CloudTrail logs API calls accessed to your AWS Account. These CloudTrail logs are stored in Amazon S3 Bucket.
The two offer different services.
The Definition you have shared from CloudTrail Doc: CloudTrail adds another dimension to the monitoring capabilities already offered by AWS. It does not change or replace logging features you might already be using.
It means you might have already activated some of the other logging features offered in other AWS services like ELB logging etc.. But when you enable CloudTrail monitoring, you need not worry about your previous logging functionalities as they will be still active. You will receive logs from all the services. So By Enabling CloudTrail logging, It does not change or replace logging features you might already be using.
Hope it Helps.. :)