Steel is a fundimental part of a kitchen knife, because obviously that is where the utility comes from. And while steel can be broken down into many, many, sub-categories, at the end of the day there are 2 main types used in kitchen knives (for the most part): Stainless Steel, and High Carbon Steel.
Here is essentially where the differentiation is between the 2 types:
Stainless steel: Holds a good edge, is resistant to rust, and is easy to maintain.
High Carbon steel: Tends to hold a sharper edge, longer, is more prone to rust and other issues, and takes a bit more effort to maintain.
In the kitchen this can make a differrence as knives are often around water, and certain types of foods that can cause issues of high carbon blades. However many chefs swear by the performace of carbon steel, and feel it is worth the extra effort to maintain it in order to have the performace benefits.
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Though the line between Stainless Steel and High Carbon Steel is slowing evaporating as new and better steels are invented with varying degree of carbon in them (All steels are techincally carbon steels), for the moment it is all a preference for the user.