"Dim" stands for diminished and what happens in these chords is that all tones besides the root are flattened. There are both the diminished triads and the diminished seventh. In the triad, the third and fifth tones are flattened one semi-step. In the diminished seventh (dim7), a minor third is added to the dim triad. Dim chords are quite uncommon and are mainly used for a transition between two chords that don't have an obvious connection.
See diagrams of dim chords: