This post contains a list of countries in the Schengen area, where you are free to move about anywhere among these countries. For Americans, this free-travel time is limited to 90 days per 180 days (3 months/ 6 months). Once you enter one of these countries by rail, plane, or foot, the 90 days starts ticking. It’s kind of like tag. Entering Austria, for example, will tag you “in” and your 90 days in Schengen starts. You won’t be tagged out so long as you travel to any of the other countries on this list, and your 90 day counter will continue to roll.
If you leave Austria and travel to the UK (a country not in Schengen), you will be tagged “out” and your 90 day Schengen counter will stop until you tag yourself back “in” by returning to another Schengen country. You will go through passport control every time you leave and re-enter the Schengen zone.
Do not test this 90-day limit and try to overstay your time in Schengen. As of this post, the following countries are in the Schengen Zone. Due to the refugee influx, some of these countries have imposed border controls regardless of prior arrangements.