There are a few key differences between Hbase and RDBMS. But before we get to that, you need to understand that Hbase acts on top of a Hadoop Server as a non-relational database management system and it operates on unstructured data whilst RDBMS is usually situated on traditional systems and acts on structured data. That being said:
RDBMS used schemas while HBase does not use schemas.
RDMBS does not support any sort of partitioning that is built into it already while Hbase does.
RDBMS stores normalized data while Hbase stores de-normalized data.