Why we are using parallel circuit in commonly instead of series circuit and also where we are using series circuit?

Parallel circuits are commonly used to powerd the electrical appliances because in parallel circuits, potential difference accross each device or appliance is same while the current through each device is different (as per rating). Kirchhoff's Current  Law is applicable on parallel circuits.

Series circuits are rearly used to powered the electrical appliances because in series circuits, potential difference is divided accross devices and the current of the circuit remains same. Kirchhoff's Voltage Law is applicable on series circuits.

Our equipment's are rated to our Country's standard voltage so if we use two devices in series rated at same voltage then we will b using2x of the rated voltage which is not available. so u have to use parallel connection circuits instead of series that's the reason we parallel circuits are commonly used. And the use of series circuit comes when u want to use2 or more devices of less rated voltage on a higher than their rated voltage.   

Parallel circuits are used where we require constant voltage supply i.e in electrical appliance 

Series circuit are used where we require constant current i.e.  charging batteries , lightning etc.

Your First Question is nicely explained By Mr Syed Qasim, regarding the second one ,You use Serial circits in illumination lighting or Decorative lighting circuits, also when there is a need of closed loop control for Eg when we need to run3 phase motors by interchanging phases to change direction , we used closed loop control in the coil supply of contactors through normally closed contacts , this is used to avoid a possible malfunction by simultaneous operation of both controlling contactors , another common example is a battery bank we generally conect them in series in case of inverters as per requirement.

Dear Mohammed, -Parallel circuits are also called power divider from which you can figure out that applications that been used starting from most off all your home appliances like power ring and light street so if one failed don't affect the rest light street maintain same voltage and controlled separately.

-While Serial Circuits are called voltage divider same current passes through resistors like light string.

Thanks 

We use parallel ciruit commonly because in parallel circuits, if fault occurs on previour accessary the fault will not affect the preceeding ones.