Difference between one way slab and two way slab: 

one way slab:

one way slab are supported on opposite sides and the load are transmitted in one direction. eg: veranda slab is a on way slab with main reinforcement in the span direction and distribution reinforcement in the transverse direction.

two way slab:

the loads are transmitted to the supports in both directions with main reinforcement provided in mutually perpendicular directions. eg: car parking with apartment

Design of one way slab:

 Reinforced concrete slabs supported on two opposite side or on all four sides with the ratio of long to short span exceeding 2. length [x axis] and breadth [ y axis ] 

FORMULA : Y AXIS / X AXIS > GREATER THAN 2 IS ONE WAY SLAB

Design of two way slab:

Multistorey building with column and beam construction, the floor and roof slabs are supported on all four sides. two way slabs should have their longer span not exceeding two times the shorter span so that so that significant flexural building develops in mutually perpendicular directions

FORMULA: Y AXIS / X AXIS < LESS THAN 2  IS TWO WAY SLAB

IS: 456-2000 CODE BOOK DESIGN SLABS