The Sign system in The Witcher 2 was clearly changed to scale much better over the course of the game than what we saw in the origianl title. Their maximum potential still depends on your character build, but all of the Signs have been made useful in some way. All of the Signs appear again in the second game, Aard, Yrden, Igni, Quen and Axii with same or similar functions. If you specialize deeper into the mage talent tree you'll also receive Heliotrop Sign. Here's a closer look at the Signs and their functions:
This Sign hasn't changed a whole lot from the first game. Essentially, it's used to stun or knock enemies down. Stunned enemies can get finished in one blow, while knocked down enemies won't be able to parry for a few seconds (approximately two blows). Aard also does minor damage and can be used as an area-of-effect spell once Geralt maxes out the "Enhanced Aard Sign" talent. The chance to stun the opponent also rises with the "Energy Flow" talent. Finally, Aard blends well with the environment as it can be used on occasions to push enemies off the walls, cliffs, and into the water. In the first game it was extremely useful, but in The Witcher 2 it requires some player skill and a couple of talents to get the most bang out of it.
Yrden has seen a significant improvement over its functionality in the first game. Its primary function is to encase or trap a foe and allow you to obliterate it with strong attacks while it sits helpless. It has a certain chance to fail versus large and resistant creatures, but it can still trap just about anything if you're persistent. Yrden can be further upgraded with talents "Enhanced Yrden Sign", which grants Geralt to place up to two or three traps at a time, and "Glyph Enhancement" which links the placed Yrden traps and prevents foes from moving between them. Should they be caught, they'll also suffer minor damage. Overall, this is a great Sign for swordfighters as it allows "free" heavy sword attacks.
This Sign is very similar to the one in the first game in the way that it becomes very powerful once proper gear and talents are acquired. The talent "Enhanced Igni Sign" adds the incineration effect to the Sign, area effect ability, and allows Geralt to turn enemies crispy before they can even touch him. Empowered with the proper +Sign damage talents and items, Igni becomes a powerhouse from about mid-game on. It can also be used to ignite the many fire sources (torches, braziers, etc) in the game.
Quen has been redesigned to be used in conjunction with sword fighting. It is widely regarded as the most overpowered Sign due to the "Enhanced Quen Sign" talent, which adds extra shield duration and reflects a portion of the damage back on the attacker, along with "Venting" talent which reflects the damage to multiple foes. The main difference from the first game is that you can attack while under the effects of the Quen shield, making this Sign extremely potent for swordfighters and casters alike.
Axii has been changed to provide a means for Geralt to invoke a type of mind control on the enemies he battles and NPCs that he enters dialogue with. With talents like "Fatal Attraction", Geralt can even control up to two or three enemies at a time. The "Enhanced Axii Sign" talent adds damage and vitality to the mind-controlled opponent, too. When using it to influential dialogue choices, it works much like a Jedi mind trick to influence the outcome of the dialogue in your favor. Aside from its dialogue properties, Axii is best used when taking on multiple monsters at a time.
Beyond the five "base" Signs, Geralt can also gain access to the Heliotrope Sign by taking the "Magical Sense" talent deep within the Mage talent tree
. This particular Sign can only be activated once you've filled up the adrenaline bar. The Sign itself slows down the time (in a way very similar to the "bullet time" you've likely seen in action games), allowing you to defeat your enemies in real-time while they move in slow motion. It's particularly useful when you're dealing with a lot of enemies at once.
As far as Sign damage and mechanics go between The Witcher 2 and its predecessor, a few things have been changed. All Signs have a flat base damage, though you can add a considerable bonus to this damage by choosing the proper talents and gear. Additionally, there's a Sign Intensity statistics that works as a multiplier on the base effect of the Sign. It is best explained with an example:
Let's say Igni deals 10 base damage, your total gear has +20 Sign damage, and your talents add another +5 damage. That would mean that an invocation of Igni deals 35 (10 + 20 + 5) damage. However, now consider that your Sign Intensity is 1.4, and that the full formula formula for calculating damage is: (base sign damage)*(sign intensity) + (+sign damage). This means that the total damage the Sign inflicts is 39 damage ([10 x 1.4] + 20 + 5). The same calculation for Sign Intensity applies for critical effects, too.
One final note to make is that circles of elements are gone. All of the game's Signs are available to Geralt right at the start of the game, with the exception of Heliotrope, of course.