CloudWatch is probably the most common monitoring service provided by AWS. It collates and collects metrics across a variety of your resources running in your AWS account. This allows you to monitor your resources' performance over time and configure and respond to alerts that meet customized and defined thresholds.
On top of this metric monitoring, you can use CloudWatch along with a unified cloud agent to collect logs of your applications running on EC2 instances and a number of different AWS services. This log's data can be read from within CloudWatch, providing you with a real-time logstream of data. Using CloudWatch Logs in this way essentially allows you access to a central repository of real-time log data.
More on Amazon CloudWatch will be discussed in Chapter 12, Implementing Logging Mechanisms.