Application performance monitoring is a set of tools and practices to introspect and diagnose large complex application performance issues with the objective of maintaining an acceptable level of uptime and responsiveness.
In traditional monolithic applications, APM solutions and libraries focused on reporting performance metrics and data points related to the process's usage of internal memory, CPU and various IO through low-level runtime instrumentation, allowing for application behavior profiling.
In today's cloud-native architecture, APM solutions aim at monitoring end-user experience given a broader set of services and interactions through Distributed Tracing.
Popular APM solutions include New Relic, Datadog, Lightstep and Splunk.