Your Peruvian apple cactus is a Cereus cactus, possibly Cereus uruguayanus (synonym Cereus peruvianus). It is also commonly known as a night-blooming cactus or queen of the night cactus. In the wild, it could grow up to 30 feet tall but indoors, in a small pot, it will probably not grow larger than 3 feet tall. Outdoors, it is known for its large, white, tubular flowers that bloom at night and last only 24 hours or less. The flowers are followed by 2 to 3 inch, magenta fruit. Plants flower only after they are several years old and it is much less common to get flower and fruit on a house cactus.

Proper care: You need to let your Cereus have the most sunlight it can possibly get. Turn it frequently so a new section faces the light or it will begin to lean toward the window. During the hottest, summer months, it will appreciate the extra sunlight it gets if you are able to put it outside on a terrace.

In the spring, summer and fall, it should stay in its sunny spot. In the winter, move it if you can to a cooler spot, as low as 50 degrees if possible, so it can rest. It may get a thin stem if it tries to continue growing during the months of much shorter days because there isn't enough light.
 
Water it moderately, allowing the top inch of soil to dry completely between waterings. During its winter rest, it will need much less water, just enough so the soil doesn’t dry completely. Too much water can lead to root rot.

You can feed it every 2 to 4 weeks during the active growing season with a tomato plant food, but it will be fine if you don’t. The food will just give it a growing boost.

Courtesy of the NYBG Plant Information Service