I have a noob question. I'm planning to buy an android phone which doesn't include sdcard slot. Only internal memory. I am now using HTC Desire S which include sdcard slot. I change ROM on my phone so often. What I'm worried about is, will formatting data before flashing ROM going to remove all data in internal memory as the one I'm planning to buy rely only on internal memory. Or does HTC make some partition for something like virtual sdcard space that is stay untouched even wiping data from recovery?
It depends on how the particular recovery you will choose to install handles user data.
Most recent versions of commonly used recovery images (e.g. TWRP and ClockworkMod) recognize the need to keep certain portions of internal memory on devices without removable storage. On such devices the
Terminology is important in this case...
Usually you don't need to actually format