Dynamically typed languages carry the types of values at runtime. That is, whatever is executing your code can look at a value itself and read off the type of it. Statically typed languages check all of the types of their values at compile time to make sure that it's sound. Then, they can strip the type information from those values and execute them without having to perform any runtime checks. In a dynamically typed language, the way to apply an operation is to read the type tag on it first, make sure the operation you want to apply can be applied to it, and then execute that operation. In a statically typed language, that check happened at compile time, so you're safe to just apply the operation to the value. Oftentimes languages may not be fully statically typed or fully dynamically typed. Java's use of reflection, instanceof, and casting means that it carries some type information at runtime. C++ has typid, C# has GetType(), etc. etc. Haskell as a language has full type erasure and can be considered fully statically typed, but there are some. GHC extensions (Typeable) which point to some type information being held at runtime by the compiler.