Until last week, Glue represented "ETL without worrying about servers." But when data scales into terabytes, when you need complex ML preprocessing or big-data ecosystem tools like Hive, HBase, and Presto, you need greater freedom and control. That's where Amazon EMR (Elastic MapReduce) comes in.
EMR is, simply put, a "managed Hadoop/Spark cluster." It automatically provisions and operates open-source big-data frameworks — Apache Spark, Hive, Presto, HBase, Flink — bundled on EC2. If Glue is "an abstracted ETL function," EMR is more like "a distributed computing cluster you operate directly." On exams, the selection criteria between these two are always a hot topic.