Overlap is the cheapest and most traditional method of wall construction for wooden sheds. Simply put it is a type of cladding where each board overlaps onto the adjacent edge of the subsequent board. These are then nailed down.
Tongue and Groove is generally considered superior and is therefore more expensive than the overlap method as it results in a more rigid structure and is better weather resistant. Using a tongue and groove profile, each piece interlocks with the previous to create a smooth finish with no distortion over time.