Minecraft wiki entry for Development_versions, associated text: Target parameters have been upgraded.
These can be added after the @ selector in brackets.
x: X coordinate of search center
y: Y coordinate of search center
z: Z coordinate of search center
r: range, 0 means no range limit
There is a short syntax for x,y,z,r options
m: gamemode, -1 means no gamemode limit
c: count, 0 means no count limit, -c returns results from end to start.
Example: @p[c=-1] will target the furthest player