SCARLET WITCH
The Scarlet Witch's powers have been retconed and reworked a number of times, in part due to her multi-layered influences, including being a second-generation mutant (and later, not a mutant at all), touched by Chthonic magic due to her birthplace of Wundagore Mountain, and the chosen Nexus Being for our reality.
Wanda's hex power originally functioned as a blast of wild probabilities -- using it caused random unlikely things to happen to her targets. Wanda could not control the hex power, and it could cause good or bad things to happen, or nothing at all.
Agatha Harkness gave her minor training in the use of magic in Avengers (1st series) #127, increasing her focus and control. The Scarlet Witch was much more powerful after this, and could concentrate on altering probabilities to create specific improbable outcomes, instead of random ones.
In Avengers West Coast #42-62, Wanda's status as a Nexus Being was manipulated by Immortus, altering her powers so they acted like retcon blasts, retroactively reordering history itself in order to produce the ever-strengthening effects of her hex power. Immortus accomplished this by feeding excess nexus energy from other realities into Wanda. When she rejected and expeled this energy, it left her temporary powerless before restoring her abilities to their previous levels.
In Avengers (3rd series) #10-11, Agatha Harkness redefined Wanda's powers for her. She told her that Wanda's normal mutant powers of energy manipulation were influenced by Chthon during her birth, causing her to naturally create and direct "chaos magic". Now aware of the true nature of her powers, Wanda could manipulate natural things (like the elements) or disrupt unnatural things (like mind control or opposing magical forces) because chaos magic was "natural magic". She could also attune herself to different energy fields to manipulate or disrupt them as well.
According to Avengers Disassembled in Avengers (1st series) #500-503, Wanda's real power was the ability to alter reality itself however she chose. Any earlier explanation of her power was Wanda subconsciously limiting herself to more reasonable power levels. Here, Doctor Strange claimed there was no such thing as "chaos magic", and the earlier Agatha Harkness from Avengers (3rd series) #10-11 was nothing more than a figure animated by Wanda's subconscious, telling her what she wanted to hear.
Then, Avengers: The Children's Crusade #1-9 has redefined Wanda's powers yet again. The events of Disassembled and her infinite power levels were only the result of Wanda attempting to tap into the Life Force of the universe, and she has now apparently reverted back to her pre-Disassembled power levels.
Most recently, revelations have come forward stating Wanda is not the daughter of Magneto or a mutant, but apparently an artificially engineered superhuman devised by the High Evolutionary. Whether the revelations of Avengers (3rd series) #10-11 still stand, and if Wanda still has the power to manipulate "chaos magick" because of Chthon's influence on her Evolutionary-intended energy-manipulation powers, is unclear.