In addition to the normal OpenStreetmap API, there is an extended API (XAPI) that gives you the ability to query OpenStreetMap data for particular sets of data. For instance, you can request data that has specific tags, covers a specific area, or is used in a particular combination of primitive elements. It's the most versatile way of getting customized sets of data from OpenStreetMap, and if you don't have the resources to set up a local copy of the OpenStreetMap database, XAPI is your next best way of getting the data you need.
XAPI doesn't run on the main OpenStreetMap servers, but instead on separate machines, so that the main servers aren't overloaded. There are several instances of XAPI running at present, each with different update frequencies, availability, and capacity.
A complete list of current XAPI servers is kept on the OpenStreetmap wiki at http://wiki.openstreetmap.org/wiki/XAPI, including details of how often they're updated from the main database. The...