Ruby is "strong typed".
Strong typing means an object's type (not in the OOP sense, but in a general sense) is checked before an operation requiring a certain type is executed on it.
Weak typed means that no checking is done to ensure that the operation can succeed on the object. (For example, when a function accesses a string like and array of floats, if no type checking is done then the operation is allowed)
Edit:
It's been 6 years since this answer was posted and I think it warrants some extra clarifications:
Over the years the notion that "type safety is a dial not an absolute" started to be used in favor of the binary meaning (yes/no)
Ruby is "stronger" typed (with an "er") than most typical dynamic languages. The fact that ruby requires explicit statements for conversion IE: Array("foo"), "42".to_i, Float(23), brings the Ruby typing dial closer to the "Strong Typed" end of spectrum than the "weak typed".
So I would say "Ruby is a stronger typed dynamic language than most common dynamic languages"
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