What is svchost.exe

Svchost.exe" (Generic Host Process for Win32 Services) is an integral part of Windows OS. It cannot be stopped or restarted manually. This process manages system services that run from dynamic link libraries (files with extension .dll). Examples for such system services are: "Automatic Updates", "Windows Firewall", "Plug and Play", "Fax Service", "Windows Themes" and many more. 

At startup, Svchost.exe checks the services portion of the registry and constructs a list of services that it needs to load. Under normal conditions, multiple instances of Svchost.exe will be running simultaneously. Each Svchost.exe session can contain a grouping of services, so that many services can be run depending on how and where Svchost.exe is started. This allows for better control and debugging.
If the process svchost.exe uses high cpu resources, it is mostly due because the service "Automatic Updates" is downloading some new Windows update. But having a 99% or 100% cpu usage could be caused by downloads due of some hidden malware on your computer. Some malware like the Conficker worm changes the Windows Registry so that svchost loads the malware .dll file. In this case you only see the authentic svchost.exe process in the task manager!