Flashing to custom ROM = Hard Reset?

[jhale83]

Ok, bear with me here as I've had Windows Mobile forever prior to getting my Hero. I've read all over trying to find this answer and I thought I found it at one point but it was a little while ago and I wasn't ready to root and flash and so forth. My question is: when you install a custom ROM on an android phone, does it hard reset your phone, or in other words does it wipe everything so you have to start over installing apps and such? I thought I read that it did NOT wipe everything but just want to be clear before I jump in. Thanks in advance for your help and sorry if I missed this somewhere in the forums.