On May 9, 2012, at 2:46 PM, Krishna Kranth wrote:
Is there any commercial restriction on using Wiki information. Is it free to use the information? Any restriction on number of requests that can be sent?
See  for API Etiquette.
As for restriction on usage, that depends on which wiki you get the information from. Every MediaWiki install has a footer. If the MediaWiki install is publicly licensed it will contain a copyright/license note.
Wikipedia, for example, says:
"Text is available under the Creative Commons Attribution-ShareAlike License."