Jon Morisi has a primer on Sqoop:
In this article, I’ll walk through using Sqoop to import data to Hadoop (HDFS).
“Apache Sqoop(TM) is a tool designed for efficiently transferring bulk data between Apache Hadoop and structured datastores such as relational databases.”
With respect to SQL Server, Sqoop has two good use cases: pulling data from SQL Server into HDFS, and pulling data from HDFS into a staging table in SQL Server.