Mkalafut did a pretty good job summarizing how the rune works, but I wanted to expand a bit more on how it compares to the other runes.
Per the Barbarian Game Guide, the Avalanche runes are:
- Volcano - Chunks of molten lava are randomly launched at nearby enemies, 4400% weapon damage as Fire over 5 seconds.
- Lahar - Cooldown is reduced by 1 second for every 15 Fury spent.
- Snow-Capped Mountain - Cave-in from both sides pushes enemies together, dealing 1800% weapon damage as Cold and Slowing them by 60% for 3 seconds.
- Tectonic Rift - Store up to 3 charges of Avalanche.
- Glacier - Giant blocks of ice hit enemies for 1600% weapon damage as Cold and Freeze them.
Volcano theoretically can do more damage, but the chunks of molten lava are smaller than the total area hit by a non-Volcano avalanche, and since you can't control where they land, in practice a given mob will only be hit by 2-4 molten chunks. If you target a large, dense clump of enemies you might come out ahead, but individual enemies won't take a lot of damage, and your standard groups of mobs and elite packs will often take less damage (since half the chunks will miss everybody). It's like taking the Death Blossom rune for Arcane Torrent on a Wizard.
In return, you're losing out on the utility of the other runes:
Lahar lets you cast Avalanche more frequently (though given the big fury costs of many spenders, I haven't found this to make a significant difference in practice).
Tectonic Rift lets you "bank" up to three Avalanches, which is nice when you have a large gap between enemies or are unable to damage your desired target for a while.
Snow-Capped Mountain summons three waves of ice at your target location and bunches mobs up - great for following up with a heavy AoE attack. Plus, the slow prevents them from un-bunching quickly.
Glacier seems to be the most popular. You gan no extra damage over the base Avalanche, but you freeze all mobs for a couple seconds. Given the size of Avalanche, this works out to be a pretty big area stun, and one that can be cast at range too.