Some of this may be wrong, this is from my understanding of working with panels, provider APIs and looking at the inner workings of providers documented by raids on the services.

.m3u: m3u was originally used as a playlist container for mp3 audio files often linked from Internet resources (like Shoutcast radio streams), but got extended for UTF-8 character set support (.m3u8) and further extended for adding additional metadata (Extended M3U, #EXTM3U). Further extended for Apple's HLS (HTTP Live Streaming) format. The end-users use of a .m3u file relating to IPTV is going to be either a list of all your available streams or a single playlist containing a single stream. Some IPTV providers flat out disable these due to how easily bruteforced the playlist generation links are, instead opting for directly talking with the IPTV panel through either an panel URL or portals.

EPG/XMLTV: XMLTV is a standard throughout the Linux TV community for implementing an Electronic Program Guide into player software through the use of .xml files. Most provider's implementations of these are garbage or subpar compared to actual EPG services like Schedules Direct because the underlying IPTV panels used by providers only keep a few fields out of the many that give a complete guide in their database.

Line: The end-user facing account that gives someone access to either an portal, Xtream API or .m3u playlist.

Stream: Usually an IPTV channel. This can also be a "created channel" that consists of video files on loop and sometimes shuffled.

Package: a selection of bouquets with restrictions that make up the fundamentals of an IPTV line

Bouquets: an arrangement of channels as part of an average IPTV package. This is not exactly user-facing unless you have some separate system to order/add/remove them from your users at will.

Categories: categorizes channels into groups separate from the bouquet and package system that could either work alongside or against them. This is user-facing.

Reseller: A middleman who works with IPTV providers so they can sell their services to prospecting customers through a middleman. They will buy a set of credits from the provider under the table and then will mark up the base price given to make a profit. This reduces risk for the provider as the front man for their services will end up as the fall guy depending on if the payments and communications between provider and reseller are secure enough. Providers will often work with many resellers. They also handle all support for the service under their umbrella.

Subreseller: Someone working under a reseller. This is the same as the reseller definition. Likely used for really big providers that only work directly with other resellers?

Enigma/Enigma2: Enigma2 is an appliance software suite for Linux-based set-top boxes. These boxes often will be able to pick up a satellite signal but also support IPTV playlists in a special format. Bouquets have a sort of different meaning on this platform; they're used for sorting providers/sources.

Portal: This is usually referred to as a "Stalker Portal". This is for Infomir's Ministra (formerly Stalker) platform. Initially they were supposed to be used with Infomir's Stalker Middleware platform but the underlying client code was reused by Xtream-Codes' and their own backend put in place. They're primarily designed for Infomir-branded MAG boxes but the APIs can be used by several independent players. These lines are often tied to MAC addresses.

Restream: The act of renting/purchasing TV channels ("streams") from a provider that deals exclusively with other providers. This is different than reselling but is the same concept. Your provider will give you information to put into your panel to pull their streams, you then are able to stream to your customers from their streams.

OTT: Over-the-top; refers to a streaming platform. This term is used for actual services like Netflix or Prime Video but apply to legitimate IPTV services like YouTube TV and DirecTV Stream. These streams are often encrypted and only viewable on a select few (sometimes requiring verification on the manufacturer's end) platforms.

IPTV panel/Middleware: This usually refers to the underlying software that makes up an average IPTV service. Usually something powered by PHP, under-the-hood FFmpeg is used to run each channel and either Enigma2, Stalker or a regular .m3u playlist is provided to each end-user line.

Panel URL: This is the URL for the "main server" used by an IPTV panel suite. Usually used for Xtream-Codes API support and refers to the base URL of your average provider (e.g. if your playlist URL is http://some.host.lol:80/get.php, the panel URL is http://some.host.lol:80/). See also: XC API.

OTT panel/streamer: These are used along with provider scripts or some other config to pull streams off legitimate OTT platforms, decrypt them and repackage them to be put into use for an IPTV panel to restream.

Xtream-Codes: Xtream Codes was a company in Italy that sold the "Xtream-Codes IPTV Panel Professional" software package. This was the most popular choice for pirate IPTV providers to run their services and just about everyone used them or were compatible. While not running any streams themselves whatsoever they ended up being tried in Naples court for piracy but ended up winning. Your average XC provider consisted of a main server and quite a few "load balancers"; the load balancing system allowed the provider to spread channels across multiple servers to decrease system load, balancing user load between streams was a secondary feature that depended on the provider to sort out. The first iterations of the panel actually fully ran on the main server including it's frontend but XC got nulled (the PHP term for software cracking, the licensing usually is straight up fucking removed) so the second version only had a backend installed on your main server and you interacted with the servers using Xtream's own frontend. When they got raided, the Italian police took down the frontend "CMS" servers which stopped every provider using XC from being able to manage lines and streams until workarounds and ultimately a new frontend was developed.

XC API/Xtream API/Xtream Code: Used by player software to refer to the Xtream-Codes player_api.php API to pull user line information, streams, TV and movie content and channel guides. See also: Panel URL.

Xtream-UI: A replacement frontend for Xtream-Codes' server-side backend (or for new installations, Xtream Codes Reborn) developed by a guy named GTA after Xtream-Codes got raided in Italy. Developed a few revisions getting up to version 22f before shelving the project for XUI.one which he then shelved a few years later. This is often still in use via the means of mods that tamper with the frontend, the backend or both to correct exploits or extend either.