"Pagan" (or, more precisely, pagan) is an umbrella term for any of a number of spiritual paths that are either A) nature based and/or B) magic based. Wicca is the best-known, and the founding of Wicca in the mid-1940s pretty much started the paganGanmmunity. Druidism as it is now is a reconstruction (of varying qualities) of what modern researchers think might have been the Celtic priesthood in Ireland prior to the arrival of Christianity.
As for which one's right for you? Only you can decide that. I recommend just taking your time with it. There's a LOT to learn out there, and no need to hurry. Learn a little bit about a lot of things, find what interests you the most, and then study those things that really capture your attention.
I'd recommend checking out http://www.witchvox.com/xtrads.html to find out more about specific traditions, as well as http://www.paganachd.com/faq/ for specifically Celtic material.
As to books, I'd recommend Nicholas Graham's "The Four Powers" (which should be available on Amazon in the next couple of weeks, or you can try Googling it) as well as Graham Harvey's "Contemporary Paganism". Both of these will give you very different introductions to a number of paths. For further reading you might also like my book review blog at http://lupabitch.wordpress.com
Good luck