To elaborate on earlier postings, Schengen countries are Norway, Sweden, Denmark, Finland, Germany, Austria, Netherlands, Belgium, Luxembourg, France, Spain, Portugal, Italy and Greece. Between these countries there are generally no border checks, though you occasionally hear travel advisories about Country X or Country Y reintroducing border checks for a limited period (e.g., I believe that Portugal was carrying out border checks during Euro 2004).

For those requiring visas there is now a single "Schengen Countries" visa, so a visa issued by one country will also get you into the others.

None of the new EU countries have joined Schengen yet, so crossing the border into Poland or Slovenia is like crossing from the USA to Canada or Mexico - i.e., you drive up to a checkpoint, you wind down your window and you present your passport (or national identity card if you are a citizen of an EU country that issues them).

As already mentioned the UK and Ireland aren't part of Schengen, so normal passport controls apply when arriving from another EU country. However, the British Isles have their own free travel area known as the "Common Travel Area" or (CTA). The five territories that make up the CTA are Great Britain (i.e., the big island that contains England, Scotland and Wales), the Republic of Ireland, Northern Ireland, the Isle of Man and the Channel Islands. I believe that in theory the CTA arrangements only apply to those who are citizens of the places concerned and that others need to carry passports when travelling - say - between England and the Channel Islands, but in practice there are never any checks.

Some non-Schengen countries have traditionally been quite lax at their road border crossings - it is fairly common to just be waved in and out of Switzerland without having to show any ID documents (though this only applies when crossing by car, there are always checks on the trains and when crossing into Switzerland on the Lake Constance ferries)