What is exadata database?
Oracle Exadata database machine is a hardware solution from Oracle which runs the usual Oracle Database Software. However it has an extension: the disks are accessed by special storage nodes which are smart: when the DB tells them to read blocks from a database table they can also tell them filter conditions: the result is that IO bandwidth and caches are distributed to multiple machines and table scans are parallelized (offloading from the main database machine). Besides that there can be additional Flash Cache and the interconnect use fast Infiniband network connections.

This makes it not only faster for typical OLAP queries, but also more robust against wild ad-hoc queries. The disadvantage is that you need to have a big workload so it is worth it.

Exadata uses Linux or Solaris as the Operating System.

It must not be confused with ODA, which is the database appliance without the special storage nodes.

Exadata is for the purpose of applications interacting with it invisible. This also means you can import backups from there and run the same queries (sans management queries for the distributed storage of course).