[Sony Xperia T, TL, TX, V] kerberizer:
This guide tries to explain very briefly the different types of CyanogenMod builds, particularly for the Sony Xperia devices, and what usage each type may be suitable for. The guide is oriented towards newcomers, who often find themselves wondering what are the differences between these various builds (don't worry, I used to be perplexed myself) and which one would serve them better. Anyone seeking further information is well advised to check the CyanogenMod wiki or search around the forums here.

The nightlies are regular automatic builds* of the CM source. Because changes get constantly pushed to the CM source from various developers, these builds reflect a more or less random state of the code, and because there's no human intervention in deciding whether to build or not, the nightlies can contain random bugs. Actually, the larger and most severe bugs are typically filtered out by the preliminary testing and review process before they even get committed, but it's impossible to catch everything that way. Indeed, the nightlies' primary purpose is just to make sure that the code still builds correctly. Nevertheless, they are also a rather useful tool for large-scale testing -- and that's how these not-so-huge bugs get caught as well.

The more substantial changes or the ones that could possibly break a lot of things (or few things really bad) get tested separately, with the experimental builds, before they are committed to the CM source.

The stable builds are based on a snapshot of the source code that is deemed suitable enough for everyday use. They are expected to be free of any serious bugs. By the way, a "snapshot" is just what it sounds like: a momentary picture of the code at some specific point of time, as decided by of the developer(s) who oversee the project development. In technical terms, the snapshot is a Git tag.

When a stable release is planned, the commits are gradually restricted to only the ones that fix things; the ones that "merely" introduce new functionality (which potentially means new bugs) are postponed until after the release (in developer terms, the code is "frozen"). In this process, the code gets several preliminary snapshots, namely, the release candidates (or RC), which reflect the advancements in the code stabilization and bug fixing. In other words, each new RC should have more and more of the identified issues ironed out, until all of them eventually get fixed, at which point the stable version is finally released (hence why an RC is just a "candidate").

The FXP builds are manual builds of the CM code with patches by the FreeXperia project. FreeXperia is the core group of developers who, with the help from other skilled developers from XDA and CM, maintain the Sony Xperia line of devices for CyanogenMod. This means that (an often asked question) most of these patches get promptly committed to the general CM source. The only exception might be changes that turned out to behave badly. In other words, any fixes or new features in the FXP builds very soon get included in the CM nightlies as well. Unlike the CM nightlies, however, the FXP builds are manual, so they are expected to be more stable: whether because the current code has been confirmed as stable enough, or because particularly problematic one has been patched. Thus, they might be considered the equivalent of the CM milestone (or M) releases.

IMPORTANT: Whichever type of build you choose, always keep current enough backups, preferably of different types (e.g. a nandroid/CWM/TWRP and a Titanium/etc. one) and in different physical locations (in particular, keeping your backups only on your SD card is asking for trouble). Keeping several chronological versions is a wise practice as well (you might, for instance, find your last and only backup to be corrupt). Remember that you can never have too many backups!

Which version should I use if I...

TBA

This guide started as a post in another thread, and I'd like to thank [MENTION=3381828]tilal6991[/MENTION] for confirming what had been only assumptions then. All your recommendations, whether to fix mistakes or to enhance the guide, or even just comments, will of course be very welcome.

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*) The current list of the automatically built targets, together with the frequency of the builds, is here.