The concept for a pilot is generally "pitched" to network executives by a producer or writer. If interested, the network will fund the writing of a script. This may happen 50 times in a particular year. At this point "development hell" commences, as various stakeholders at the network propose changes, and rewrites occur to satisfy those proposals.
If the script for a pilot has satisfied the stakeholders at the network and is sufficiently exciting, then the production of the pilot itself can begin. Perhaps 10% to 20% of the scripts commissioned by Hollywood networks actually get to the production stage.
Pilots are expensive to produce. Before a network commits to funding an entire pilot episode, it often requests a pilot presentation, a one-day shoot that, when edited together, gives a general idea of the look and feel of the proposed show.
Pilots most often run in the fall television season, but also after the winter holiday hiatus, when mid-season replacement shows air to replace those shows cancelled mid-way through the year. More rarely, a pilot will air in summer, as was the case with the 2003 pilot of the U.S. show, The OC.