See here for a brief overview of TVS diodes
http://en.wikipedia.org/wiki/Transient_ ... suppressor
And here for a datasheet about these types of diodes.
http://www.st.com/stonline/products/lit ... n/5628.pdf
Also a good one.
http://www.littelfuse.com/data/en/Appli ... an9768.pdf
[edit] good as well, titled "How To Select Transient Voltage Suppressors"
http://www.microsemi.com/micnotes/125.pdf
[edit] also good, titled "Protecting Data I/O Ports With TVSarraysTM"
http://www.microsemi.com/micnotes/121.pdf