How does a currency get 'weaker'?

Question

like the dollar, nowadays.

Answer 1

due to inflation

inflation is a sustained and consistent increase in prices in all the markets of the economy because of the aggregate demand and supply.

and due to scarcity

Answer 2

Currencies - like most goods and services - are bought and sold.

Hence a currency has a demand and supply curve where the price can be measured in any other currency.

If there is excess supply of a currency (ie people want to sell it more than others want to buy it) the price will fall.

The dollar is weak now because there is excess supply - largely driven by the public and private sector debt crisis. Essentially the US, both the government and the citizens, is spending more than we are earning. To do this we have to borrow - and so the difference has to be borrowed from overseas.