The two main parts of your eye that focus light onto your retina to enable you to see clearly are the cornea and lens.

The cornea is that transparent, dome shape in front of your eye and your lens is located inside your eye and changes its shape to allow you to see things at varied distances.

So, What's the Problem?

You'll need different pairs of glasses for distance and reading.

Well, as you get older your lens becomes less flexible and reduces your ability to see things clearly up-close, at which point you'll need reading glasses.

This is because there are muscles around your lens that contract and make it change shape to enable light to be correctly focused on your retina. 

In children's eyes, the lens is much more elastic and can easily change shape to cope when  switching from far to near objects. Whereas, as we age, the lens becomes stiffer, this means it is less able to alter its shape and thus we're less able to see those close-up objects. Also, this means that switching focus can also be much longer as we get older as well.

Because the lens has lost its elasticity, you need glasses to make up for it and focus on the different distances you need to see.

But, What's the Cure?

Basically, there isn't any, it's part of the process of getting older. Though one of the main ways around it is to wear reading glasses as it's near sight that tends to be affected first, so you'll see people pearing over the rims of their glasses to see things far away - they will look blurred through the glasses. You can also use bifocal glasses with two separate lens areas - one to focus light from objects far away and the other for reading.

You can also find that reading is more difficult at night and things seem more blurred or even when looking at text where the contrast is low and in low light. This is because your pupils get bigger (dilate) in dimmer light and have less focal depth. Whereas in bright light you have increased depth of focus and don't notice the blur as much.