NoSQL and BIG DATA

Question

I am doing an internship on Big Data technologies so I am new to this area. My question is about the use of NoSQL in the Big Data architecture. Do we need always to use a distributed storage (like HDFS in the case of Hadoop) then to put on top a NoSQL databases (like Hbase )?

I find it difficult to understand the typical BIG Data architecture specially for unstructured data .

If you can help me to see better I will be so thankful.