Dennis: curved track is measured by the radius -- from the center of the circle to the centerline of the track. The smaller the radius, the sharper the curve.
16" radius is considered an absolute minimum for HO gauge trains, unless you go for smaller than usual. Even wider is better if you like modern long cars, but sectional track doesn't usually come in widr radii.
The other measurement you may see is degrees - that tells you how much of a circle each curved piece takes. There's 360 degrees in a circle, so a 30 degree piece means 12 to a circle; 22 1/2 degrees means 16 to a circle. (Often they don't tell you and you have to guess. trying to make a circle from 12 pieces of 22 1/2 degree curves gives a lot of kinks.)
