Strength in Go is measured by a system similar to the one used in martial arts. In fact Go is considered to be a martial art in the Asian countries.
A complete beginner starts from about 35 kyu (35k) and as progress is made this number is reduced. When a player reaches 1k he/she is becoming ready to get the first dan (shodan).
There are 7 dan levels for amateur Go players. Professional Go players rank on top of these dan levels, and there are 9 additional dan levels which professionals may attain.
The amateur rating systems are not consistent over the world (see
comparison chart). For example it is said that European and American dans are stronger than Japanese dans.
People who play in tournaments in Europe, including us in Cyprus, get the opportunity to be included in the European Go Federation rating system. This can be accessed
here. Similar systems are maintained in the United States (by the American Go Association) and in other countries.
There is a rating book for use when teaching children, developed by German Go player Kalli Balduin. Such a book is used in Cyprus by Dimitris Regginos. You may check it out
here.