I saw another post this morning regarding machine needles. While I was searching for something else I went out to the Schmetz website and found tons of good information, so I thought I would pass the link on for those who might be interested.
http://www.schmetzneedles.com/all-about-needles/
Here is the link to the Needle Primer which is a great quide to the differences in the types of needles.
http://www.schmetzneedles.com/schmetz-needle-primer/