There are three types of Ultra Violet Light. UVA, UVB and UVC.
Light is measured in a unit called nanometers (nm). Our eyes are only able to perceive light from about 400nm to 700-750nm. Light wavelengths above and below this are not seen. Below would include UV rays, above would include things like waves from the radio or a microwave.
Sunglasses that provide protection from UVA and UVB are identified on the inside frame of most sunglasses. If not on the frames themselves, there is usually a tag with them.
UV 400 means that all UV rays are blocked, so even the tiniest UVC rays aren't getting through. This is what you want to see when you purchase sunglasses.
The other important thing to remember is that clouds do not filter UV rays, so even on cloudy days you need to wear sunglasses to prevent damage.
The long term damage to your eyes can include cateracts and macular degeneration. So if you enjoy spending tiem in the great outdoors, make sure you always wear sunglasses with sufficient UV rating.