If a user needs to get your vector data, for example the USA railroads, he can use the Web Feature Service (WFS) protocol. It is a standard protocol defined by OGC that refers to the sending and receiving of geospatial data through HTTP.
When delivering data, the most important thing to define is the data format. Vector data is usually stored in a binary format--think of a shapefile or a PostGIS table--but for practical purposes, we need a more standard approach. Indeed, WFS encodes and transfers information in Geography Markup Language (GML) based on XML.
There exists a few versions of WFS and GML. The current GeoServer release supports the 1.0.0, 1.1.0, and 2.0.0 WFS versions.
You can find the full reference for WFS and GML at the OGC repository at http://www.opengeospatial.org/standards/is; look for OpenGIS Geography Markup Language (GML) Encoding Standard...
