What is Porosity

"Porosity" is the measure of pore spaces of a porous media. Mathematically,
Porosity = (Bulk Volume - Matrix Volume) / Bulk Volume = Pore Volume / Bulk Volume

where
Bulk Volume = the total volume occupied by a given rock body in 3D
Matrix Volume = the volume occupied by the rock grains
Pore Volume = the volume occupied by the space in between the rock grains.

Figure-1

In Figure-1 the outer black rectangle represents the "Bulk Volume", "Matrix Volume" or sand grains and between the  "Pore Spaces" or "Pore Volume" are represented in brown and white color respectively.

It is represented by the Greek letter ϕ (phi).

"Total Porosity" is the ratio of total pore space irrespective of weather they are connect or not to the bulk volume. "Effective Porosity" is the ratio of connected pore spaces to the bulk volume. This is the actual pore space from where fluid can flow to the producing wells (Figure-2).

Figure-2