1) SerDe is a Library which is built-in to the Hadoop API
2) Hive uses Files systems like HDFS or any other storage (FTP) to store data, data here is in the form of tables (which has rows and columns).
3) SerDe – Serializer, Deserializer instructs hive on how to process a record (Row). Hive enables semi-structured (XML, Email, etc) or unstructured records (Audio, Video, etc) to be processed also. For Example If you have 1000 GB worth of RSS Feeds (RSS XMLs). You can ingest those to a location in HDFS. You would need to write a custom SerDe based on your XML structure so that Hive knows how to load XML files to Hive tables or other way around.