All digital images have a certain resolution that the camera will save the photograph at. It's the important thing to remember if want to make your photograph bigger, if you want to blow it up.
A camera like this, a high-end DSLR, the pixel resolution is going to be somewhere around 7,000 pixels by 5,000 pixels. Keep in mind the majority of the internet is about between 800 and 1,000 pixels wide for most images. So, 7,000 by 5,000 is a lot of detail to work with. But again, you have to remember that photographers are using this for magazines, billboards, and things like that because they need that pixel resolution.
Something, for instance, like an iPhone, which I have right here, is going to have a pixel resolution closer to 2,500 pixels by 2,000 pixels. The reason for that is the sensor in this camera is much, much smaller and has much less resolving power than the sensor in this camera. Resolving power just relates to the actual resolution that you can pull out of an image.
Again, if you're only posting your photos on the web, you're not going to need to worry about resolution so much. If you want to get them printed large, it's important to remember that you need to be using a camera that is going to have enough resolution to allow you to do that, and an iPhone might not be your best choice.
That's just some basics about pixel resolution.