Dilution plating - viable count (only living cells)
The sample is diluted by the serial dilution technique and then the dish containing the most colonies and with no clumping is chosen and the colonies are counted, to find the total viable cell count the number of colonies is multiplied by the dilution factor.
In this method each dilution decreases accuracy so an average is taken. Should count the smallest dilution with no clumping as the clumping could lead to an inaccurate count and underestimate. As only living bacteria grow colonies - assume each colony is produced by one bacteria.
Haemocytometry - total count (living and dead cells)
In this method you introduce a standard volumes of bacteria solution into the haemocytometry - a glass slide with lots of grid lines.
This is then placed under a microscope and the number of cells is counted, not possible to distinguish between dead and living cells, more accurate method than dilution plating.
Turbidmetry - total count (living and dead cells)
Uses a colorimeter to measure the cloudiness or turbidity of the culture as cell members increase. The light absorbed is recorded and can be compared with a standard graph of light absorbance plotted against known cell numbers to estimate the number of cells in the solution.
The more turbid (cloudy) the solution is the more cells are present.