One Way Slab:
One way slab is the slab having ratio of longer span to shorter span equal to or more than 2. In one way slab main tensile reinforcement is placed parallel to shorter side which will be the main steel.
Ly/Lx > 2 [one way slab]
One way slab = longer span/Shorter span > 2
Two Way slab:
If the ratio of longer span to shorter span is equal to or less than 2, then such slabs are called as two way slab.
Where as in two way slab both direction required main steel.
Ly/Lx < 2 [two way slab]
Two way slab=longer span/shorter span < 2