Types of corrugated board
Single Face corrugated board consists of one ply of fluted paper, onto which paper or cardboard is glued: 

Single face corrugated board
Single Wall (double face) corrugated board consists of one ply of fluted paper which is glued between two plies of paper or cardboard:

Single wall corrugated board
Double Wall corrugated board consists of two plies of fluted paper which are glued together by one ply of unfluted paper or cardboard and exposed outer surfaces of which are each covered with one ply of paper or cardboard. Double and triple-wall corrugated board is also produced for high stacking strength and puncture resistance: 

Double wall corrugated board
Flute Sizes
Common flute sizes are "A", "B", "C", "E" and "F" or microflute. The letter 
designation relates to the order that the flutes were invented, not the relative sizes. Flute size refers to the number of flutes per lineal meter, although the actual flute dimensions for different corrugator manufacuturers may vary slightly. Measuring the number of flutes per lineal foot is a more reliable method of identifying flute size than measuring board thickness, which can vary due to manufacturing conditions. The most common flute size in corrugated boxes is "C" flute.
 Flutes per lineal meter
 Flute Thickness (mm)