Bulbapedia has an extensive article on it, but it boils down to, in Generation III and on, there is no difference in stats between normal Pokemon and shiny ones.
Generation II (Gold, Silver, and Crystal) is the exception because shininess is determined based on the stats of the Pokemon. Namely:
If a Pokémon's Speed, Defense, and Special IVs are 10, and its Attack IV is 2, 3, 6, 7, 10, 11, 14 or 15, it will be Shiny
But, from Generation III and onward, it is determined based on a secret ID and a trainer ID, so it has nothing at all to do with the stats and will be just like every other Pokemon.
Something fun to note is that if the Male/Female ratio is 7:1, in Generation II, you cannot have a female shiny Pokemon because of the way it is calculated.
The two terms are orthogonal - an expansion pack might be DLC, but that would imply two different facts:
- It is downloadable, e.g. it can be obtained entirely via online means and installed automatically into the game.
- It adds significant features into the game.
DLC - "downloadable content" - is a very broad term for any additional parts of a game that you can obtain from any of the various online vendors. It's generally reserved for official content from the original publisher, to distinguish it from community content. That is, an additional module for Skyrim to add fancy armor for your horse would be a "mod" if a user did it, but "DLC" if Bethesda did it. DLC can also be very big; for Skyrim, both Dawnguard and Hearthfire are DLC and they make extensive changes to the game.
An "expansion pack" is a separate product that builds on top of an existing game to add significant new features. Expansion packs can usually be purchased at retail and installed from CD, and exist separately from their parent game. They are typically "mini games" in their own right, but rely on the content and engine of their original game for much of their content. In Skyrim, Dawnguard is obviously an "expansion pack" - it add new lands, new weapons, new skill trees, etc. I don't know if it's available for retail yet but I would be very surprised if there's not a Skyrim GOTY edition that includes the two expansion packs.
A better example might be Oblivion: there was a handful of DLC released, including horse armor and Knight of the Nine. Later, there was an actual expansion pack - Shivering Isles - that added an entire new land and new features into the game. (The line was blurried a bit when Bethesda packaged all of their DLC onto a CD for retail sale, but it was basically the same as downloading the DLC and burning the archives to to disk.)
With so much game delivery being online these days, and with official downloadable content getting bigger, the line between a "plug-in" and an "expansion pack" is blurring, and all of it would be considered "DLC".
Best Answer
In short, Mythical Pokémon are a subset of Legendary Pokémon.
A Legendary Pokémon is characterized by its rarity, its inability to spawn offspring of its own evolutionary chain, and its presence in the lore. Usually, only a single Legendary Pokémon of its species exist in a game, and it is encountered by interacting with it outside of battle or by other special means. Some examples of Legendary Pokémon are Articuno, Raikou, Heatran, Mewtwo, Thundurus, and Zygarde.
A Mythical Pokémon is a Legendary Pokémon whose main method of acquisition is out-of-game, real world distributions such as online events or codes given at specific stores. Mythical Pokémon cannot be encountered by just playing through a game. Some examples of Mythical Pokémon are Mew, Victini, Jirachi, Arceus, and Volcanion.