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Like database store information for ESX, Cluster, Resource POOL etc..or something else.
vCenter database will have all the infomation what ever you see via vSphere Client, Datacenter, Cluter, VM, vSwitch details, datastore vDS, Performace etc..
Check out the below guide to know what all the information is store in DB
http://www.vmware.com/files/pdf/vc_dbviews_41.pdf
Also if my database crashed then VC will not work. If I reinstall (not restore from backup) the database what information I will get when I will login to VC.
If not restore from backup then its like fresh install of vCenter and you need to perform all vCente related configuration and you will lose all performace statistics