[ Correction:  Prior to 2007-NOV-05 some places in this guide mistakenly used the term "radius" when it should have said "diameter".  This has now been corrected.  I apologize for any confusion.]

There is a subtle and sometimes confusing difference between O and O27 track and trains.  I hope this guide will help you to understand something about the different types of O Scale track.

Historical O gauge tin-plate track has curves that form a 31" circle.  At some point Lionel wanted to create a smaller and/or less expensive train set, so they "invented" O27 track.  This track has curves that form a 27" circle, hence the name, "O27".  Note:  That is the letter "O", not zero. 

O27 track has rail and ties that are slightly smaller and lighter, although the distance between the rails is the same in O27 and regular O.  The original, heavier track is sometimes called "O31" or "O-31"; more commonly, however, it is just called "O gauge".

Today, both O and O27 track are available in a number or diameters including 31", 54" and 72".  There is also Lionel's Super-O track which has brown plastic ties, more realistically shaped running (outside) rails, and a thin, "invisible" center rail.  This was an attempt by Lionel to appeal to those who wanted track that looks more like a prototype railroad.  Super-O curves form a 36" circle.

There are also a number of O gauge track systems, more recent than Super-O, offered by various vendors.  Some examples include MTH's RealTrax, K-Line's Super-K, and Lionel's FasTrack.  These are offered as "sectional" track.  That is, they come in sections of various length and radius.  In this regard they are similar to traditional O and O27 sectional track, but, like Super-O, they have a plastic base that simulates realistic ties and roadbed.  They use different and (unfortunately) incompatible methods of joining sections. 

There is also O gauge flex track, most commonly Gargraves.  This comes in 3-foot sections (actually 37") and can be bent and cut to whatever curve and length you want.

Gargraves also makes S-gauge and Standard Gauge flex track.; and they make sectional track compatible with their flex track.  A number of manufacturers make S-gauge sectional track.

For O gauge trains, the defining limit is the DIAMETER, not O vs. O27 track construction.  Trains that are designated "O27" can negotiate the sharper 27" diameter while some O gauge trains will require the wider, 31" diameter.  Some O Gauge trains require a diameter even wider than 31".

Both O and O27 trains should be listed in the O Scale section of eBay.

Here is a photo of several sections of O gauge track.  From Left to Right:  Tinplate O; Tinplate O27; Super-O; Gargraves flex track.

All of this only applies to "toy trains", using three-rail track.  O scale track is also avaiable in two rail versions.  These are generally used by people who are trying to create true scale models that are as accurate as possible.  In general scale models intended for two-rail track will not work on three-rail, toy train track.  Similarly, O Gauge toy trains usually do not run on two rail track.