It depends on which particular phone you are using. Some phones are simply not able to be flashed. Others can flash very easily.
The most common system for use in flashing custom ROMs on phones is called CyanogenMod. For a while, there was actually a CyanogenMod app in the Google Play Store that could be used to automate the process, but Google removed it. You can find more information on how and whether your phone can be flashed at
(See related link)
Note that flashing your phone usually voids the warranty, and may brick your phone. You should probably not attempt to flash your phone if you do not know what you are doing.
Watch this video tutorial cnet.com/videos/how-to-install-a-custom-android-rom/ to see how you can install a custom ROM in Android.
As a rule of thumb, it's a good idea to wipe or reset your phone prior to flashing a custom ROM. It's also a good idea to save all your files and data to your phone's external SD or to your desktop as well before the wipe. After you do that, then flash that custom ROM. If you choose to flash the custom ROM without doing a wipe (called "dirty flash"), you might encounter bugs in your everyday usage.
You can't jailbreak an Android smart phone but, you can root an Android device. Basically when you root an Android smart phone or tablet you can copy your app files and get custom ROM.
Assuming your phone has been rooted and a custom recovery app is installed, you can flash any kind of custom ROM. Some users will install different custom ROM's until they find one that they like. Others will install "nightlies" which are beta versions of a ROM that's not fully stable yet, but are updated daily.
You must root your phone and flash a custom ROM on it. Note that, while rooting has many benefits, it will void your warranty. Flashing a custom ROM is a little more involved, but XDA-Developers has all the information you'll need to get Gingerbread running on your Wildfire.
A custom ROM is modified software, typically of an operating system that you would overwrite or erase over the phone's stock system. The customized ROM may have features that may be different or better than was was on stock.
The Kurio 10s will generally be able to be flashed with a custom Android ROM but one should consult the workers and employees of the store before proceeding.
Nothing really "happens". You still have the same phone but just not Android Jelly Bean. Your phone company may release an update for you or a custom ROM may have been created online.
You have 2 choices: 1)Your carrier released a new version of android for your phone. 2)You root your phone and flash a different ROM into your phone.
Officially the "max update" is Android Gingerbread. Developers are working a custom ROM with Android 4.0!
It's possible if the apps package is backwards compatible to earlier Android versions.
From my own experience, yes, you so can. I might warn you though, it needs rooting your device, which can brick your phone. I'm not responsible for any damage, so here what I do. Example: You have an android phone with Android 2.2, and you want android 2.3 gingerbread. Before doing anything else, go to XDA developers forum, and search for you device and the custom roms. If you see any android 2.3 roms in there, then go in, and read the comments to make sure that it's working on other phones too. If people are commenting with "thanks", "it's great", "perfect ROM" or any other positive feedback, then you can root your phone, and put that custom ROM on your device.