I will be traveling in Europe from May 1-Aug 1 2008. Does this pass give access to all trains in the 16-17 contries or just to the main cities?
Virtually all trains in Eurail countries are covered. You do have to reserve (and pay a reservation fee for) many of the fastest trains like the TGV, and you do have to pay for sleeping accomodations on overnight trains. There are also several private (but spectacular) trains in Switzerland not covered by your pass; there may be discounts for passholders there too.
Western and central European countries not on Eurail include the UK, Czech Republic, Slovakia, Poland, and countries of the former Yugoslavia; some have their own country passes. It also doesn't get you on the Eurostar to London, though I believe there is a discount for passholders. Go to the "Railpasses" tab at the top of this page for a more thorough
explanation of options.
Determine the number of "Travel Days" before purchasing your Rail Pass.
Some point-to-point tickets are cheaper than using up a Rail Pass day.
Once you have a tentative itinerary you can go to this site and enter all the pertinent information. Be sure to use the proper European spelling of the city names. You can find those on Rick Steves' web site. This site will suggest the Rail Passes you need or advise you to purchase point-to-point tickets.
http://www.railsaver.com/
Your Rail Pass should be the last thing you purchase. We got an extra day when we purchased ours in Dec, Jan or Feb forgot exactly which month Rick offered the extra day.