Distinguish between AC and DC.
Alternating current (AC)  Direct current (DC) 
The direction and magnitude of AC changes.  The direction and magnitude of DC is constant. 
AC have power factor between 0 and 1.  DC always have a power factor of 1. 
AC have finite frequency.  DC have a zero frequency. 
AC is obtained from alternator.  DC is obtained from battery, solar cell, DC generator etc. 
The passive parameter is impedance denoted by Z.  The passive parameter is resistance denoted by R. 
The direction of electrons flow changes forward and backward.  The direction of electrons flow is steady i.e. forward. 
There are different waveform types of AC like sine wave, square wave, triangular wave etc.  There are only two types in DC one is pure DC and second is pulsating DC. 
Step up and step down of AC is very easy using transformer.  Step up and step down of DC is not very easy because we can not use transformer with DC. 
Transmission cost of AC is low as compared to DC.  Transmission cost of DC is high as compared to AC. 
We can easily obtained DC from AC using rectifier.  We can obtained AC from DC but the inverter circuit is difficult. 
We can not store AC.  We can store DC in battery. 
Generation of AC using large alternators is economical.  Generation of DC using large DC generator is not economical. 

Particulars 
Direct Current 
Alternating Current 
Definition 
It is the current whose magnitude and direction do not change with respect to time. 
It is the current whose magnitude and direction continuously changes with respect to time. 
Use of transformer 
Not possible 
Possible 
Design of machines 
Complicated 
Simple 
Frequency 
Zero 
It is 50 Hz or 60 Hz depending upon country. 
Obtained from 
Battery, Cell and DC Generator 
Alternator 
Passive parameters 
Resistance only 
Resistance, Inductance, Capacitance. 
Applications 
DC machines, HVDC system, electroplating, Battery charging, Traction. 
AC machines, Domestic and industrial supply. 