At first it is important to know then when you hear someone talking about “full frame” or “APS-C” that the person is talking about the sensor of the camera. In fact about the size of the sensor.

The APS-C sensor is smaller than a full frame. That to you means that if you put a lens of for example 50mm on a full frame camera you get actually 50mm but if you put it on a camera with APS-C sensor you have to multiply it with a crop factor, which is 1.6 for Canon and 1.5 for Nikon. So on a Nikon APS-C camera the 50mm will look like 75mm and on one by Canon like 80mm. 

Now there are different ways to think about this crop factor. If you want to zoom in a lot for example when you are doing wildlife or sports photography you will be happy about every available mm of focal length that you have more. But if you want to shoot wide angled for example in landscape photography then this is a disadvantage for you because you will be grateful for every mm that you can go wider. 

Full frame sensors are equivalent in size to 35mm film and they are generally going to perform better at high ISOs due to their larger photosites.

This is a very important thing to think about when you are going to buy your camera! Think exactly about what you need for the type of photography that you are going to do!

After all you need to be aware of the fact that at first place when you are buying a full frame camera that it will cost more than an APS-C camera. But also that the lenses will be more expensive! Because not all of the cheaper lenses are compatible with the full frame sensor!