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What are roms?

You have probably often heard that getting root allows you to flash "custom roms", but what exactly are they? First of all, ROM stands for Read Only Memory, and is essentially the entire system of your phone. When you first get your phone, it contains a stock ROM (notably sense for HTC). 

Custom ROMs have many benefits compared to stock roms

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  • Selection
there are multiple custom ROMs for each phone, giving you the choice to pick what suits you
  • No bloatware
though this is essentially the case as soon as you root, custom roms generally don't come with useless apps, of even Google apps itself for that matter. (side note: flash Gapps with roms)
  • Update frequency
this is a big one. No more waiting months for one single update. With custom roms, updates can happen up to once a day. Scroll down for details.
  • Battery/Performance
another very big benefit. ROMs can be made to make the most out of your phone, and often come with various tweaks to make your phone last longer and perform better
  • Size
if your phone doesn't have an external memory slot like my Htc One S, you'd value space quite a bit. Sense based roms can be extremely big (~700MB), whereas custom roms based on AOSP (android open source project) can be as small as 130MB.

Which rom should I flash?

This is one of the most common questions posed by newcomers. Over at xda, they generally keep rom preferences at a minimum, as it does depend on not only the phone model, but the phone itself. If you want to find the best for your phone, try them all and pick the one that suits you. Nevertheless, I'll give my experience on the ROMs I've tried. Don't want to read and just want a list? Here you go.
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Probably the most common of the bunch, CyanogenMod is known for its popularity and stability. If you are a newcomer, your best bet is to try this first. This is also the base of many other custom roms.
Personal note: Does what it says. Is quite stable (though I had issues with cm10.1)
An acronym for Android Open Kang Project, AOKP is known for its love for unicorns. This is another one the popular roms, and is absolutely loaded with features.
Personal note: Didn't use it much but did indeed benefit from its endless customizations.
The glorious makers of Halo! This rom is also packed with features.
Once you get into the rom flashing, you're likely to miss some roms for their features. That is not a problem, as PACman compinces Paranoid android, AOKP, and CM together along with all of their features (PAC is an acronym if you haven't found out yet).
Personal note: This one drained my battery, though that is expected with all the features. I loved tablet mode
Love the color black? Slim is all about lightweight roms with blackouts. Their gapps are fricking amazing.
Personal note: Though I used an unofficial version, I absolutely loved the stability. Carbon 4.3 was a very good rom for me. Featured pack and lightweight.
Essentially CyanogenMod but stripped down to the core, Codename Lungo takes pride in its lightweight. It also boasts faster performance and better battery life.
Personal note: I used it and didn't see a major improvement. Might be just me though
Made from the people who once were a part of CM, these guys started something new. They allowed bug reports even in nightly, and had the multiwindow feature.
Viper, Trickster
Never used them since they didn't really go pass android 4.2, but they are sense based roms with lots of customization
Never used it but I'm just putting it here because it is quite common.
There are also many other roms that come and go, as well as unofficial versions that are ported from XDA users. I've seen illusion, chameleon, liquidsmooth (this is decently popular), bakedbean, odyssey, twistedkat, beanstalk (popular) etc. The list is endless.

Terms you should know

These are the daily updates. They come with the newest features but also have the most risk. As they are new, it also means that bug reports are generally disregarded. Use at your own expense. Do note that this isn't as scary as it seems and that many nightlies can be quite stable.
A monthly build that gives a more stable version.
Release Candidate
Just under stable, this build basically has all the bugs fixed and is a "release candidate" for stable
The most stable of them all with virtually all errors fixed. This probably marks the start of a newer version build.
Full wipe
Aka factory reset, clean wipe. This is when you wipe cache, dalvik, system, and data from your recovery (before flashing a rom). Once you do this, all your apps/data is gone.
Dirty wipe
When going from different builds (ie nightlies) of the SAME ROM, you may do this. This is the same as a full wipe except you don't wipe data, meaning all your apps/settings are saved. This may cause issues, and you should do a clean flash if errors and f/c are to occur.
Force close. Basically when x service has stopped, making your phone unusable