[Desire HD Q&A, Help & Troubleshooting] 50chickens:
an application must support multitouch and other gestures. eg, opera doesnt support multitouch (ie you cant zoom in and out) but the browser that comes with the DHD does.

boot loader = turn phone off. then, volume down + power on.

application must support running from /sdcard. When you install an app go to settings -> Applications -> manage applications. From here you can uninstall or move apps to the sdcard. Bear in mind if you have not rooted your phone you may not be able to uninstall some apps (eg amazon reader).

some application data is stored on your sdcard, and apps are stored here if you have moved them by using the steps above. if you change sd cards this info is unavailable to the app but the app should still work. any settings you had need to be redone.

the kernel is not part of a rom or the radio. It is stored in the boot partition in the emmc (where fastboot(aka hboot)/splash images/boot(kernel)/recovery/ are all stored). It is generally included with but not required by a custom rom. ie, a custom rom builder may choose to include a specific kernel with his rom but it is not mandatory.

yes, you can do OTA HTC updates. but, there are no custom (non signed) updates delivered OTA. An unrooted rom will not be able to flash non signed updates (or roms).

there is a good thread here - http://xdaforums.com/showthread.php?t=841076.

android application. eg, visionary+.

hboot is also known as bootloader. it is the white screen with the skating androids when you do the volume-down + power combination when switching on your phone.

s-off stands for security off. you can check the status by doing the volume-down + power combo. At the top it will either say s-on or s-off. If you have s-on then your bootloader (hboot), boot (kernel) /system (android) and recovery (eg clockworkmod) partitions are all read only.

clock work is the recovery partition. you have 3 possible options before loading android when you do the volume-down + power on combo. hboot, then either fastboot or recovery. fastboot and recovery are both ways to flash images to your phone although through different ways (fastboot uses the fastboot command on your pc to send the image, recovery uses .zip's on your /sdcard). clockworkmod is a custom recovery image which you need to flash after achieving s-off. it's also where nandroid backups can be done. if you select recovery before doing this you will get the phone + red triangle icon. thats the stock recovery. the clockworkmod is the green menu selection one.

see above.

clockwork mod can backup your phone's partitions (boot/userdata/system) so that if you mess up installing a rom you have a way to recover. this is known as a nandroid backup.

adb is android debug something. It is a way to send commands to the android operation system from your pc (either windows or linux). eg, you can run a single command, or run a shell (terminal session). it is part of the android sdk under tools.

busybox is a single executable whihc runs under the android OS that provides standard linux commands (eg, ls, rm, cd) to any application. it is so that a script writer does not need to know the partiticular rom you have and then cater for it, but only that you have a certain version of busybox. eg, busybox 1.17.1 provides the same commands whether it is on a leedroid rom or a cyanogen rom.

fastboot is a way to flash images to your phone. it is accessed via hboot. you need the fastboot exe from the htc developer site. it can flash the boot (kernel), recovery (eg clockworkmod) and the radio (i think).

the emmc is the storage device which holds hboot, fastboot, recovery, /system, /userdata and (not sure about this one) radio. when you switch on your phone the emmc is read-only due to the s-on flag being set by hboot. you need to flash the engineering hboot which does not set s-on when you switch your phone on.

ext2 and ext4 are linux file system types. eg, windows is ntfs, linux has ext2/3/4 and others.

rom manager is an android app available on the market place to help with the flashing process. it can flash recovery partitions (as long as your phone is s-off), make nandroid backups (as long as you have clockworkmod & s-off). it can also flash custom roms. it does not flash these from within the android OS (afaik) but it will reboot into (not sure on this one) the recovery partition and leaves it instructions to flash a particular rom.