What is aps-c format in camera?

It's the most common format in modern DSLR (Digital cameras), it's cropped 1.6x the full frame sensor (professional cameras like the canon 5D mark II for example), and the final image will be zoomed at that rate (1.6 Full Frame), you'll find this format in all entry level cameras, and even in some semi-pro ones (like the canon 7D used for sports or animals photography)

Advanced Photo System type-C (APS-C) is an image sensor format approximately equivalent in size to the Advanced Photo System "classic" negatives of 25.1 × 16.7 mm.

Full-frame cameras size as a frame of 35mm film(26x24mm)

APS-C is a format of DSLR Sensor which is  also called crop frame.

if you use  50mm lens on a crop frame camera the focal length of your camera will be changed to approximately 80mm lens.

the crop factor is x 1.6.

what ever lens you use on crop factor camera it doesn't matter just multiply with 1.6 to find out the  actual focal length.

crop sensor has more noise in dark situations. They are cheaper than full frame.

because its actually smaller then full frame.

How ever no matter APS-C or Full frame. but the composition does matter.

Full-frame cameras are called such because they have a sensor that's the same size as a frame of 35mm film (26x24mm). APS-C format cameras are called that because their sensor is approximately the same size as the Classic format on the smaller APS-C format film.