2-7/8-inch gauge
Joshua Lionel Cowen's first train, The Electric Express, ran on 2-7/8-inch track. Lionel made trains in this gauge between 1901 and 1905.

Standard gauge
In 1906,Cowen announced his new "standard gauge" (2-1/8 inches wide), which Lionel manufactured until 1939. (Standard gauge was eclipsed by smaller and less expensive O gauge, which we introduced in 1915 and produce to this day.)

OO gauge
Lionel OO gauge is 1:76 scale and very close in size to HO. It was manufactured by Lionel between 1938 and 1942.

HO gauge
At 5/8 inches wide (with a scale of 1:87), HO is exactly half the size of O gauge. Lionel made HO gauge from 1957 to 1967. (It's the most popular gauge on the market, but a bit small for young hands.)

Other gauges in use today include:

TT (Table Top)
TT is 1/100 scale and (we've heard) popular in Russia.

N Scale
N is 1:160 scale and the second most popular scale after HO.

Z scale
This is the latest gauge from Europe, with a scale of 1:220.