True cactuses are succulents, but not all succulents are cactuses. 

To be a cactus, a plant must be member of the Cactaceae family; plants in this family have a combination of characteristics--including stem features and flower and fruit structure--not possessed by other plants. Additionally, the spines of cactuses, nearly always highly visible, are arranged in clusters (although sometimes these appear to be solitary spines). Spine clusters aren't found in other plants that could be mistaken for cactuses.

There are over 6,500 species of water-storing plants that are succulents but not cactuses. They belong to 44 different botanical families that grow around the world.  All succulents, including cactuses, have evolved special water-storage tissues in thickened or swollen leaves, stems or roots. Many have waxy tissue that reduces evaporation from plant surfaces.

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- Courtesy of NYBG Plant Information Service