Can anyone explains the differences between the jquery versions (jqueryUI + jquery mobile) and when each one can be utilized?

Is it possible to build cross platform web app works on  pc's browsers as well as mobile devices using jquery technology? if yes which one to be used jqueryUI or jquery mobile?

Are there any company/site provide jQuery ready templates/theme where can be used in Web Apps (I'm saying web app not web site) where in web app a lot of fields for database usage etc.?

Is there any place/site where I can find freelance developers have web app templates or interface and would like to sell it with reasonable price, can contact me on my email adscnet@gmail.net?

Any info .. well be helpful.

Thanks in advance