ADF Overview

It is an data orchestration service provided by Azure. That can be used to perform various ETL operations
It has support for 100+ connectors

Allows to create pipelines to schedule data driven workflows
Build complex ETL processes that transform data visually with data flows
Allows to use Azure Compute Services like (Databricks Clusters, HDInsight, Azure SQL DB)

Annotations

Annotations are similar to the tags feature in Azure
There are used to identify pipelines in ADF
They are additional information that can be used to filter and search for data factory