How is steel different than iron?How many different kinds of steel are there?What type is the strongest?Which type is the weakest?
Steel has a higher carbon content than iron. The remaining questions are covered by a whole chapter in any materials text. In short, there are many classes of steels and steel alloys each with its own properties and heat treatments to yield a specific strength and hardness value.
As first answer says, if you look at the number of commercial steel alloys available and consider that any given alloy can be heat treated to a wide range of physical properties, there are thousands and thousands of potential combinations. Technically, steel is an alloy of Fe and C but there are Fe-C alloys that are called cast irons, not steel, and... there are lots of alloy steels which have significant amounts of other elements added like Cr, Ni, Nb, V, Mo, etc. Fe alloys that have a lot of Cr and or Ni added are called stainless steels and there are dozens of them and many of them can be heat treated to produce a wide range of properties. As far as the strongest or the weakest, you have to get really specific about exactly what you mean because some steels are designed for room temperature properties, some are designed for elevated temperature properties, some for static loads, some for impact loads, some for wear resistance, etc, etc.. Steels make up the largest family of metal alloys (by weight and by volume) that humans use. There are a number of reasons for this but the big reasons include: 1) there is a LOT of iron on earth 2) it is relatively cheap to produce 3) you can easily change the physical properties over a every wide range. As an example... you can take a piece of steel that is so brittle it will shatter if you drop it on the floor and heat treat it so you can bend it like a pretzel without cracking and then heat treat it again to make it very strong and tough (resistant to fracture).
steel has carbon in it instead of just iron there are hundreds if not thousands of different kinds of steel for different applications strength depends on the definition. some have stronger tensile (pull) strength than others.
