Economics of Utility

This is the measure of usefulness, satisfaction, e.t.c
Maginal utilty measures the utility gained from an extra unit consumed
On most goods the Marginal Utility falls as you increase consumption of that particular good.
For instance the MU of you first chocolate bar will be high, the second less and so on

In general goods are not free. To maximse total utility, consumers will arrange their consumption, so that the ratio of MU to P is the same for each good.
E.G. the last pound spent on each item gives the same utility.

MU tea = MU bread
P tea = P bread

It is important to understand that the Total Utility differs between items e.g. Total Utility of water is greater than the Total Utility of diamonds. However the Marginal Utility of an extra litre of water is close to 0, but the Marginal Utility of your first diamond may be quite high.
If you were in a desert the MU of water would be high and diamonds low
Diminishing MU explains why demand curves slope down. To encourage you to buy more chocolate bars the price needs to fall.