Can some body help me in downloading some portion of hybrid maps of google as I want to store them in my server and display them on web application using openlayers so that the user will only have access to only that particular area, not the whole map
Google does not allow caching of tiles. But there are sources for tiles you can use, such as Open Street Map, and Open MapQuest. But you'll need to download and store lots of tiles depending on the size of your area and whether you allow zooming. Open Street Map has limits on the number of tiles you can download in a batch. Open MapQuest is a bit more flexible.
You can limite the map extent of map by passing a maxExtent to the constructor.
Could not understand the reason for download since a web app.
Since Google doesn't allow users download tiles and you're using a server for your web application, why don't let your server download the part of google map your user is interested to each time he needs it?
Or maybe try to explain better you request, probably you want to make some kind of offline maps application, I don't know...