what are dog years?

It is not an exact value.  Generally it is thought to be 7years.

Say a human is expected to live for 70 years and a dog is expected to live for 10 years.

A one year old dog is equivalent to a 7 year old human

An 12 year old dog is thought to be the equivalent of a 12*7=84 year old human.

Here are a couple of related web pages

http://dictionary.reference.com/browse/dog+year

www.pedigree.com/all-things-dog/dog-age-calculator/default.aspx

A dogs age i think

Maybe the number of years a dog lives are called dog years!