That is the Charge Bar. The Charge Bar encourages you to use normal attacks, as it the fastest way for you to charge them and improve the effectiveness of the your Active Skills, where you have to manage your limited mana pool.
The amount of damage that you need to inflict to fill your charge bar scales with your level. If a low DPS weapon for your level will charge slower than a high DPS weapon for your level.
There are Active Skills such as the Embermage's Blazing Pillar (not entirely accurate skill, but good for building charge in grouped up mobs) that builds your charge at a faster rate, and there are skills that don't accumulate any charge (e.g. Magma Mace, Firebombs, Firestorm, and Hailstorm).
Each Charge Bar for each class works in a different manner:
With Berserker, when it's full, you will enter the frenzied state. You will run faster, attack faster, and always crit until the frenzy ends. It's worth noting that the frenzied state countdown doesn't begin until you hit something (after it's been filled up). You could potentially beat up that practice dummy in town to get your frenzied boost and then port into your destination and start the fight with a boost. The charge starts decaying a few seconds after combat and drains at a slower rate than the Outlander's charge bar.
The Outlander's charge bar works a bit differently compared to the other classes. As you fill it up, you will get the passive bonuses (Up to +10% Cast Spd, + 10% Dodge, +10% Crit, +10% Atk Spd) you get. There doesn't seem to be a break point threshold for each +X% bonus gain. The more damage you do, the charge increases, but it's also constantly decaying, so it's pobably the slowest to fill out of all the classes. Filling up the gauge up has no particular effect unlike the other classes. Additionally, it's worth noting that an Outlander with no charge will deal additional damage and stun the next enemy unit they hit. This bonus stacks with bonuses from shotgunnes.
The Engineer's charge bar has specific charge points (up to 5) and that a large number of their abilities use up in order to activate, and/or are more powerful (longer range/more damage) based on how many charges you have. The bar fills and drains much like the Outlanders, but a charge point seems to stay full longer after the charge point has been completely filled.
Embermages are very straight forward, when the gauge fills up, the Embermage goes into a concentration state (for 12 seconds) where all skills cost no Mana and they do 25% more damage on top of any bonuses (tiered, passives, etc.) they've invested in. It's best to save your more powerful mana heavy skills for this state.
I think all the character customization options are available -- at least that's how it appeared to me as I booted up the demo for the first time last night. There's also a randomization feature if you just want to click that until you get something you like.
I also like the fact that each class can be male or female and that when sending your pet to town you can also include a limited shopping list that he'll fill (potions and identify and town portal scrolls).
I believe the demo is so large because it includes the entire game but, of course, it's locked beyond the demo part until you pay for it.
Oh, one other addition/change from the first game -- you may now "fish" with dynamite. Not very sporting, but faster -- though I think the instructions for using dynamite to fish say you won't be able to catch as many fish from a given fishing hole as you can with a fishing pole.
Best Answer
Looking through a number of forum posts, Red Wolf will always look for up to two targets to savage when you crit. It's not a chance - the skill is just worded poorly.
TL;DR: Between the two skills you're looking at, Rage Retaliation is more reliable than Red Wolf. However, your best bet for picking a skill is to try fully investing in each and doing a respec if you don't like the results.
Red Wolf is not reliable because it hits the two targets closest to your character model, not your target. If you are using a long reach melee weapon, you will find that you rarely proc Red Wolf because you are standing too far from your target. This is even more true when using ranged weapons, which will only savage enemies standing right beside you when you shoot from a relatively long range. Some players have claimed that even standing close they sometimes do not proc Red Wolf, possibly because hitboxes are used instead of a distance check. It also doesn't help that Red Wolf seems to have an internal cooldown, meaning it won't proc every crit depending on your attack speed.
For best results with Red Wolf you need a short-ranged melee weapon. Then you should see two enemies being savaged every time you crit. With any longer ranged weapons you will have to stand closer to your target.
Given usability issues with Red Wolf, you will likely see better results pumping points into Rage Retaliation. Rage Retaliation:
Rage Retaliation will only feel weak:
If the above two points are a deal-breaker for you, then don't bother with Rage Retaliation. Also note that neither skill is at maximum effectiveness against a single mob.
Follow-up forum post, and original forum post.