It isn't a cactus. Cacti have spines. Your plant is definitely a succulent, however, noted by it's thick, fleshy leaves.
It looks like a member of the
Crassula family, perhaps
C. perforata. If it is
C. perforata, the plant needs more light, though, because the leaves should be "stacked" closely together on the stem, not separated as those on your plant are.
However, there are literally hundreds of species and varieties of Crassula, so I could be totally wrong about the species. For that matter, it could be something other than
Crassula, but I really don't think so. (MUST get my new glasses asap!

)
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams