Ratings

In Go all players have a rank, which describes their strength. These ranks go from a total beginner at 30kyu (pronounced as the actual letter "Q") to 9dan professional..

1.Kyu - You start at 30k (shortened version of 30Kyu) and work you way down to 1k, which is an experienced player.

2.Dan - After 1k the proceeding rank is 1dan, and at this point instead of going from 30k to 1k, you now go from 1d to 7d.
Meaning a 20Kyu is weaker then a 10Kyu but a 5d is stronger then a 1d.

A Difference in rank would be called a "stone" so 10kyu is 5 stones weaker then a 5kyu.

3.Proffesional - There is another level of ranks that are professional level. However you can only receive the title of a professional rank in Japan. Even though there are many amateur Dan players that can beat a professional player the professional rankings are supposed to be above the Dan level players.
1d, 2d,...,6d,7d->1p,2p,...7p,8p,9p.

International Ranks

Just to make things confusing, there are various differences between what strengths different ranks signify in different countries. Here's a summary:

Summery from www.gobase.org

* USA: as described above

* Europe: generally 1-2 stones stronger than US ranks (a European 1-kyu might be as strong as a US 1 or 2 dan, and European 5-dans have the same range of strengths as US 6 dans.

* Japan: generally 1-2 stones weaker than the US. The Japanese Go Association confers amateur diplomas for a fee, and the professionals who give the examinations tend to err on the side of generosity.

* Korea: Totally different. Koreans tend not to start keeping track of their numerical rating until they're quite strong (maybe the equivalent of low kyus in the US). 1 gup in Korea (the Korean word gup is related to the Japanese kyu) is as strong as a US 6 dan, and amateur dans are (last I heard) only given to the very strongest players who win amateur tournaments.