as the title says... is it just like reformatting or you can do more stuff when you flashed the n800? i also want to know the pros and cons if there are any?

thanks in advance...

Flashing is just like reformatting, it's not that you can do more stuff, just that you have it clean given that you wiped the internal and external cards.

Pros: Feels like a brand new N800 with the preloaded apps
Cons: Preloaded with unwanted apps

Depends on whether or not you're flashing on upgrade. It's basically analogous to upgrading the OS on your computer. OS2005-OS2008 are sorta like Windows 98, 2000, XP, and Vista (well, more like OS 10.2-10.5 since it actually gets better with each upgrade ), and the intermittent releases are like service packs for Windows or point-point upgrades for OS X.

No, a plain-old flash of the same firmware release you have now will not provide you with improvements (other than starting you off with a clean slate), but a flash of a newer firmware release than the one you have now will make things better (since it's an upgrade).

ohh ok... thanks... but what does it mean if people says "flash to RD mode"?

Use the flasher tool to set the device into R&D mode. Generally speaking, you'll know it if you need it.