A digital picture is made up of thousands of dots. The count of how many dots in an image is known as the resolution. Generally, what defines the quality of an image is the resolution, the higher the resolution of a picture, the clearer and smoother the image will look.
Print & Screen
Resolution of printing is measured in DPI (Dots Per Inch), the most common print resolution for a printer is 300dpi, so in a square inch you will find 90,000 dots.
A screen resolution tends to be 72dpi, although technically measured in PPI (Pixels Per Inch).
Our eyes notice the resolution of print more than we do of a screen, partially because we sit further away from our screen than we would normally with a print out but there are a few other factors too.
Digital Cameras measure their resolution in Mega Pixels or MP for short, this is a count of the total dots used to make up the picture.
If printing at 300dpi you will be printing around 7.9MP on an A4. For optimal quality you would need a 7.9MP camera, any more would gain nothing unless printing on larger media. That said, you will still get a good quality print from a lower resolution camera.
It is important to note that when using a camera the lens also has a large impact on quality of image, typically camera phones have a low quality lens which can cause photos to look grainy.