Vertices are nothing more than coordinates on the map. Linedefs and segs reference vertices for their start-point and end-point.
The vertices for a map are stored in the VERTEXESlump, which consists of a raw sequence of x, y coordinates as pairs of 16-bit signed integers. The bytes are in little-endian order (least significant byte first).
Due to the 16-bit signed two's complement format, each coordinate can range from -32768 to +32767.