[Q] A beginner needs help!

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http://forum.xda-developers.com – Hello.. I have SE Xperia Arc with ICS 4.0.4 \ Build.No : 4.1.B.0.431 -- [by official PC Companion updating program] , original ROM , and with LOCKED BL. Firstly, I would like to know if there is anyway to keep/restore my programs AFTER unlocking my BL ? Please explain (as I don't even know how to flash using fastboot xD) . And, does using custom roms need "rooting"? If not,does it erase any data? Finally, what in the world is a "kernel"!? And,why do we need it !? Any help will be appreciated. Thanks in advance. (HowTos)