Hadoop Counters Explained:
Hadoop Counters provides a way to measure the progress or the number of operations that occur within map/reduce job. Counters in Hadoop MapReduce are a useful channel for gathering statistics about the MapReduce job: for quality control or for application-level. They are also useful for problem diagnosis.
简言之，Counters是监控MapReduce运行状态的工具。
Types of Hadoop MapReduce Counters（分类）:
Ref: Hadoop Counters | The Most Complete Guide to MapReduce Counters