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In most cases "im" and "in dem" mean exactly the same.
However, "in dem" puts emphasis on the direct article, which applies that the following noun refers to a specific or a unique object or person, while "im" doesn't have this connotation. The following example will illustrate the difference:
"Ich habe den Film im Kino gesehen." - "I saw the movie (not at home, but) in the cinema."
"Ich habe den Film in dem Kino gesehen, in dem wir uns kennen gelernt haben." - "I saw the movie in the cinema where we first met."
A sentence like "Ich habe den Film in dem Kino gesehen." sounds unnatural, because the emphasis on the direct article without specifying why "Kino" is specific or unique makes no sence.
