First and foremost, this is a development forum, not Q&A. If you need to ask a question such as "how do i...", "what does...do/mean", etc go there. If you have a question about a specific aspect of a specific ROM, ask in the thread about it. If you have the ROM there will be a thread about it unless you just made it. Now onto the FAQ.
Q. What is android?
A. Android is an OS for smartphones, more specifically it's a version of Linux. There are several versions to date and they all have names as well as version codes. The most recent (at the time of this post) are Froyo(2.2), Gingerbread(2.3), and Honeycomb(3.0).
Q. What is root?
A. Short answer: If you don't know, you don't need it. Long answer: root is equivalent in Linux to having administrator privileges in Windows. It allows you to change more system settings as well as run apps that need root access. It also allows you to flash custom ROMs as well as do a lot more. Root is for power users, not your average user. Can the average user benefit from root? yes but power users will benefit more. Root can also screw your phone if you don't know what you're doing.
Q. What is a ROM?
A. A ROM, simply put, is software for your phone. It's the main OS and usually includes other apps, tweaks, settings, and the like you don't normally find in a stock phone.
Q. Where do I get ROMs?
A. Look in the board for your specific phone.
Q. How do I flash a ROM?
A. First you need root. You also need some sort of custom recovery. I prefer clocwork so that's what I'll describe. Other recoveries should be similar. Reboot your phone into recovery, go to "Install zip from sdcard", pick "choose zip from sdcard", then pick the zip file containing your ROM. Allow it to flash, flash any other zips you may need (gapps, kernels, etc), then reboot your phone. Flashing a new ROM resets many aspects of the system so the first boot could take up to 5 minutes. It is also recommended to do a full wipe when flashing a ROM unless you're updating from an older version of the same ROM.
Q. How do I flash the GApps zip?
A. Same way you flash a ROM.
Q. How do I flash a kernel?
A. Same way you flash a ROM.
Q. How do I flash a radio?
A. There are a couple of ways. Some radios come in zip format, you can flash those the same way you flash a ROM. Some come in .img format, you need to flash those from fastboot (S-OFF required)
Q. Where do I get a radio?
A. Check the forum for your phone. MT3GS users look here
Q. What is S-OFF?
A. A security state for the phone. Having S-OFF allows people full root access to the phone, even when in the main OS. With this you can add/remove system apps without the need to boot into recovery or reboot after removal/addition.
Q. Do I need S-OFF?
A. Ask yourself these questions: Do I know what I'm doing? If yes next question. Do I want the latest/greatest thing? If yes next question. Can I recover my phone if I completely destroy my OS? If no then you don't need it. If yes next question. Can I recover my phone from a full brick, not a semi-brick and/or do I develop ROMs. If yes go ahead and get S-OFF.
Q. How do I get S-OFF?
A. Look in the forum for your specific phone. For MT3GS users see here. I suggest you read the last 10 pages or so before attempting so you don't ask a repeat question.
Q. My phone shows a red triangle, what do I do?
A. You're in stock recovery, press volume up+power to get the menu. Ignore the cache error.
Q. How do I get root?
A. See the forum for your specific phone. For MT3GS users see PC Method and the No-PC Method. They use the same exploit so they're very similar.
Q. What is a kernel?
A. A kernel is the core of a Linux OS, it's what all the other software is built around. No core/kernel, no OS. A custom kernel enables features that the stock kernel didn't have such as OCing.
Q. What does OC stand for?
A. Orange County. Just kidding :P OC stands for overclock, taking your phone's processor and making it run faster but work harder. It can cause stability issues and reduce your phone's lifespan so evaluate if you need it or not. Also make sure to test the boundaries your phone can handle. Some can go higher than others without issue.
Q. How do I get into recovery?
A. That's different for each phone. For MT3GS power off the phone, press volume down+power. You should boot into HBOOT, Press volume down to highlight recovery the press power to select. If your phone has Rom Manager with Clockwork installed you can also boot into clockwork directly by choosing "boot into recovery".
Q. (MT3GS Specific) I'm using alpharev's S-OFF and my phone freezes what do I do?
A. When it says to press "any key" press and hold power on your phone, then press any key on your keyboard.
Q. (MT3GS Specific) I can't get alpharev's to boot.
A. You either burned the image improperly, have a bad download, or don't have your BIOS setup to enable booting from removable media. Try the LiveUSB version posted in the thread (one by me and one by another user).
Q. (MT3GS Specific) In the PC method you posted I can't find the files! Help!
A. There are links to the files in the No-PC method. They'll work.
Q. What is "cooking" a ROM?
A. Building a ROM from source is usually referred to as "cooking" a ROM, but sometimes cooking can also mean rebuilding a themed ROM.
Q. Where can I find a good android kitchen?
A. Search XDA, search is your friend.
Q. What is a "kitchen"?
A. The kitchen is the environment that is used to build a ROM.
Q. How do I cook a ROM?
A. That depends on your kitchen, it should come with instructions.
Q. Does this count as development?
A. I think so. It discusses flashing custom ROMs, gaining root, cooking, theming, recovery menus, and the like.
Q. Android Rocks!
A. Yes, yes it does.
Q. How do I install a custom boot animation?
A. Move the bootanimation.zip file to /data/local on your android device.
Q. How do I install custom boot sounds?
A. Move the android_audio.mp3 file to /data/local on your android device.
Q. What's the best ROM? (thanks to SefEXE)
A. Short Answer: Check the ROM Bible for your phone. Slide ROM Bible Long Answer: There is no best ROM, only ROMs that are best for you. Look through the different ROMs and see which one fits your needs the best.
Q. My phone isn't seen in recovery in the loop, help!
A. That method is old. That's why I didn't link to it, it no longer works.
Q. My phone is in a boot-loop, what do I do?
A. Give it 5-10 restarts, if it hasn't booted into your OS by then it's probably not going to so you need to reflash your ROM. (expanded on by fermunky) Try removing the SD card to eliminate something on it being the culprit. Next, if this was after trying to install a custom ROM, either try to re-install it, or if all else fails, do a wipe and re-install. This assumes you are able to get into fastboot/hboot/recovery from a boot loop.
Q. Do I need S-OFF to get root?
A. -_- no. You need root to get S-OFF.
Q. Do I need S-OFF for X?
A. You only need S-OFF for changing the /system partition while booted in android, flashing things through fastboot (splash screens,radios, etc), and custom MTD partition sizes.
Q. How do I install flash/does flash work?
A. Flash isn't supported on our phones, it's a hardware limitation. Use the flash lite plugin.
Q. How do I revert to stock after rooting?
A. That depends on your phone, you may be able to just flash a stock ROM it may be a little more involved than that. MT3GS users look here.
Q. I can't find the marketplace after installing a custom ROM.
A. Custom ROMs are not allowed to have the market pre-installed. You must download the latest gapps zip file and flash it.
Q. What is a2sd/app2sd?
A. It is a mod for ROMs that allows installing apps to your sdcard. Some ROMs have this built in by default so check before looking to install it.
Q. What is a2ext?
A. Similar to a2sd it allows you to install apps on your sdcard but uses a Linux ext partition instead of a FAT/FAT32 partition.
Q. How do I find my Radio version?
A. This can normally be found in the HBOOT/FASTBOOT screen of your android device. Getting to this screen depends on your device so the methods vary.
Q. How do I find my Android version?
A. Go into settings, scroll down to "About Phone", then look at "Android version".
Q. Should any ROM work with the newest Radio?
A. Yes, the Radio should offer many improvements over the old one and not cause any stability issues. You will, however, want to flash the latest released radio and unroot before returning your device for servicing. The latest Radio is not always the latest released Radio.
Q. I have a question about CM7.
A. Go here. Some things are repeated from here, others are not.