What is AWS CloudWatch?
One word: Performance Monitoring
AWS CloudWatch is a monitoring service to monitor your AWS Resources and the application that you run on AWS. CloudWatch is used for Performance Monitoring. It can monitor EC2, Autoscaling Groups, Elastic load Balancer, Route53, EBS, Storage gateway etc. It can also monitor Host Level Metrics like CPU, Network, Disk etc.
CloudTrail vs CloudWatch
CloudTrails is like a CCTV and used for increasing the visibility for API Calls and CloudWatch Monitor Performance.
CloudWatch Alerting: Yes, you can create alerts in CloudWatch as well and trigger notifications for it.