What does “King of the Hill” mean in Overwatch

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".

Lanes are the main roads in game where people come and fight. Those are usually the larger roads on the map, where the objectives (like towers) stand, and where the monsters spawning in your base (creeps in Dota, minions in League of Legends and Smite) face the ones from the other team. Those are also the roads which are linking the two bases.

This term appears in many MOBA games. As you mentioned, there is Smite, but that's also the case for Dota or even League of Legends (what we call "Top", "Middle" and "Bot").

For League of Legends, these are the lanes :

This is the same concept for Dota :

For Smite, those are the larger roads on the left, middle, and right on this map :

The alternative paths that we can see on the maps are usually not called lanes. Those are just alternatives roads. Usually, that is what we call the "Jungle". There is also, in League of Legends and Dota, a road which divides the map in two diagonally : the "River". The traditional monsters of your team never use those roads.

Not all the maps necessarily have three lanes. For instance, the Howling Abyss map from League of Legends, or the Smite Assault Mode's map only have one lane. Another example is the Twisted Treeline map in League of Legends, which has two lanes (on the top and the bottom).

There are also maps without lanes. Smite's Arena Mode for instance does not have any. But most classic maps, where the goal is to take down objectives (such as towers) and go to the other player's base, contain at least one lane.

(Credits to League of Legends Wikia, Quora and Smite Academy for the pictures).