What are $DC$ signals and $AC$ signals? Why do we prefer an $AC$ signal?

(N/A) $DC$ signals (current or voltage) do not change their direction with time. They are unidirectional signals.
If the voltage obtained from a source varies like a sine function with time,then such a voltage is called an alternating voltage ($AC$ voltage).
The current driven in a circuit by an $AC$ voltage is called an alternating current ($AC$ current).
$AC$ is preferred over $DC$ for the following reasons:
$1$. $AC$ voltages can be easily and efficiently converted from one voltage level to another using transformers,which is not possible with $DC$.
$2$. Electrical energy can be transmitted and distributed over long distances much more economically using $AC$ compared to $DC$.