How many layers of data can be stored on a DVD?

On the market in the DVD format currently are DVD5 (Single Layer) and DVD9 (Dual Layer). DVD5's are the most popular, hoever if you own a DVD with a movie that exceeds 120 minutes in length then chances are it's a DVD9. There is also DVD18 on the market which are Double-sided- Dual-layer DVD's and can hold 17GB of data compared to 8.5GB on DVD9 and 4.7GB on DVD5.