Euphorbia mammillaris variegata
(This plant comes in a 3.5" pot. You will receive a very similar plant to the one in the picture. It may or may not be blooming at the time of your purchase. The plant is shipped in its pot to prevent any damage to the roots).
Origin: South Africa (Little Karoo and Southern Cape area)
It is an easy species to grow that is suited for any well drained soil in full sun. But young plants are happy growing indoors, where they can easily reach the ceiling. Water regularly during the active growing season from March to September. No water should ever be allowed to stand around the roots. Keep almost completely dry in winter. It is a moderately fast grower, and will quickly become large landscape masterpieces in just 3-5 years. It can tolerate moderate shade.
Blooming season: Late winter to early summer.
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz