Mutant Ability to control Chaos Magic
Initially, the Scarlet Witch had the ability to manipulate probability via her "hexes" (often manifesting physically as "hex spheres" or "hex bolts"). These hexes are relatively short range, and are limited to her line of sight. Casting a hex requires a gesture and concentration on her part, though the gestures are largely a focus for the concentration. Early in her career, her hexes were unconscious on her part, and would be automatically triggered whenever she made a particular gesture, regardless of her intent. These hexes would only manifest "bad luck" effects. She later gained enough control over her powers that her powers only work when she wants them to, and they are not limited to negative effects. Despite this enhanced precision, her hexes are not necessarily guaranteed to work, particularly if she has been straining herself or using her powers excessively. If overextended, her hexes can backfire, causing probability to work against her wishes or to undo previous hexes.
Her hexes have a wide, almost limitless variety of recorded and possible effects, though they often boil down to a kind of Deus Ex Machina superpower. They have been known to alter the molecular composition and physical state of physical objects, negate or distort physical laws, and to cause various forms of energy to spontaneously appear or disappear. She has a natural affinity with natural elements and phenomena, stemming largely from her magical training and origins, and has trained often at using her hexes to deflect projectiles or to cause enemies to stumble or otherwise suffer the effects of "bad luck". Writers often confuse her hex powers with psychokinesis, and have occasionally depicted them as able to generate energy blasts and to grant Wanda the power of flight. Although her hexes can cause objects to suddenly move or change course, they do not have the ability to "hold on" to objects as a telekinetic might. Ultimately, the hexes are not under the Witch's direct control, although she can influence the nature of their effects with great concentration and effort. This makes her one of the few Avengers that Ultron fears; his adamantium shell cannot repel magic. She has displayed the ability to cancel Longshot and Domino's powers of probability and has also once turned Longshot into a cat.
The first major reclassification of Wanda's fairly-indistinct hex powers came during Kurt Busiek's run on Avengers, where her mutant power was claimed by Agatha Harkness to be an ability to manipulate chaos magic, given to her by the demon Chthon (imprisoned within Wundagore Mountain) when she was born. Her hexes were, by extension, simple, indirect manifestations of this magic, destabilising probabilities by inducing chaos. Across Busiek's run, Wanda's powers grew continuously, as she gained the ability to manipulate organic matter and summon Wonder Man back to life. While in the DC Universe, she displayed the ability to access that universe's chaos magic, but had great difficulty in doing so at first, since it was more powerful there and she had no previous experience in wielding it.
The second, current redefinition of Wanda's powers came during the Avengers Disassembled crossover event, where they were revealed to be an ability to manipulate reality. Her probability-manipulating hexes have been redefined as the conscious manifestation of this subconscious power. The idea that Wanda possessed this level of power was first suggested by Immortus during his conflict with the Avengers West Coast, although Immortus specifically claimed that Wanda could manipulate the past to influence the present. Her powers were used by Morgan Le Fey to alter reality in Avengers (volume 3) #1-3, although this was only accomplished with magical amplification. It was in Avengers Disassemled that the full extent of this power came to light, when it was revealed that Wanda had been subconsciously altering the Marvel Universe to an unknown degree for a number of years, including the reanimation of the deceased Agatha Harkness without consciously realizing it to be the case (which goes some way to retconning away the previous "chaos magic" origin, as it was not truly Agatha who explained it, while Dr. Strange's claim that chaos magic does not exist, is contradicted by but one brief moment when he himself has employed it in the past, in a fill-in issue, during an era where he was using "catastrophe magic".)This retconning has caused much debate and disgust in the comic fandom recently.
This aspect of her power is not under her conscious control, and continues even after Dr. Strange shut down her mind. There doesn't seem to be any limits to her powers as she was easily able to alter the entire 616 reality to create the House of M, and is also altering reality on a multiversal scale without even realizing it!
The source of Wanda's reality manipulation powers is due to the fact that her mutation allows her to tap into magic. This was confirmed in Avengers Disassembled by Dr Strange, and by Dark Beast in Excalibur, who said that magic is just another way of manipulating forces on a quantum level. It has been suggested that Wanda's powers have been influenced by the demonic magician Chthon: in particular, that Chthon's own magic redefined Wanda's power to have a similar function, although this remains dubious, having been claimed by the faux Harkness.