There are various gauge of steel strings. "Lights" are commonly known as "10's" based on the thickness of the treble E string. The string gauge is actually 0.010 inches. Extra Lights would be 9's. This would be for electrics. On acoustic, a "light" steel set for an acoustic would have a treble E of about .011. Although exactly what gauge is referred to as "light" can vary.

There is a loose standard about what gauge is light, medium and so on, but it's best to choose specific gauge so you aren't subject to the vagaries of convention.