Sidewalk
A sidewalk (also sometimes footway/pavement/footpath/platform) is that part of a highway set aside for the use of pedestrians, often separated from the carriageway (or roadway) by a kerb (also curb).
How to map
For much of the current OSM database there is no information provided about the existence or not of sidewalks which can make it difficult to provide good pedestrian routing, and in particular good narrative descriptions of pedestrian routes.
In most places in the current OSM database (June 2011) there is a general assumption that pedestrian access is possible for all roads other than motorways unless the foot=no tag is used to indicate that pedestrian access is prohibited. This tag does not however say anything about the existence or not of sidewalks and the practical options for pedestrians.
The simpler method to described sidewalks is to tag the associated highway using sidewalk=both/left/right/none for those sections of sidewalk that are parallel with the carriageway and then use highway=footway where the path diverges from the carriageway. This approach is simpler for routers to support but cannot currently provide fine details about pedestrian obstructions (including barrier=* and kerb=*s) or for tactile_paving=*. A rendering issue is that the point where a pedestrian route diverges from highway at an angle is mislocated due to the width of the rendered road or has a small part at a wrong angle.
The more detailed and time-consuming approach is to map each sidewalks as separate ways using highway=footway and an associated footway=sidewalk with (footway=crossing used for places where the footway crosses a street. This method allows for a more spatially accurate representation of the pedestrian environment and allows the use of barrier=*, tactile_paving=*, kerb=*. The method works with existing routing engines and renderers but tends to produce a more cluttered map. Future pedestrian routers will need to associate the footway way with the highway to produce narrative directions (e.g. "Follow the sidewalk along Main Street"), which will require grouping the separate ways by e.g. using a relation or surrounding both ways with a area:highway=* area.
Some might say that the separate way method is more suitable for rural/suburban areas, while the sidewalk=* method is better for urban areas.
