OpenStreetMap data includes “tags”, name=value pairs which provide a free-form folksonomy approach to classifying features. To get started mapping you don’t need to know about these. With a easy to use “edit” interface, you can simply select from a range of feature types. For example if you select “post box”, then a node will be created with the “amenity=post_box” tag set on it.
But as you learn to contribute data, and certainly if you try to use OpenStreetMap data, you’ll most likely need to understand tags. To help you do this, there are a number of resources available.
You can search the OpenStreetMap wiki for tag documentation such as the Tag:amenity=post_box page. This documentation is created by the community, but rather than regarding this as the final word, understand that these wiki pages form part of a process to reach agreement on tags. You and everyone else in the community can help edit those pages to ensure the documentation is more exhaustive and more accurately reflecting the tags used by the mapping community.
But for a more direct view how tags are used by the mapping community, we have taginfo. This system counts up the ocurrances of tags in the OpenStreetMap database, and presents these statistics to give us a feel for which tags are most popular, and how many of these features we have mapped. For example, just now we see the amenity=post_box tag appears 153581 times in our database!
taginfo also mines the wiki for information, finds icons used in JOSM, provides links to Overpass Turbo, XAPI, and JOSM launcher, and generally brings information about tags together to connect different sources and users.
This system was developed by Jochen Topf several years ago, but building on similar ideas such as TagWatch and TagStat which came before it. These days taginfo is a crucial tool for understanding tags, and it has been rehosted on OpenStreetMap Foundation servers with various changes described by Jochen on his blog