The best description i have seen, by Andreas Antonopoulos:
Dapps are applications that cannot be stopped, as they run in a decentralized plataform.
And why should someone run an app like this? If you have an application that somehow offend someone powerful, or is against their interests, then dapps are tbe way to go.
Ethereum blockchain was build specifically to run smartcontracts and dapps
so is this mean even though a single person has multiple nodes still cant get the control over it !! or is there any logic
Nobody has control over it.
The idea behind dapps and smartcontracts , on ethereum, is to have applications or contacts which none of the parts control.
The dapps will be running as long as the conditions are met. Running multiple nodes makes no difference, because the dapps will be on all nodes.