what is the difference between a solenoid and a coil? and also tell me that what is the nature of magnetic field inside a solenoid when a current is passed through it?

There is no particular difference between a coil and a solenoid. A solenoid is a long coil containing a large number of close turns of insulated copper wire. Its shape is somewhat like a spiral loop of wire. It may be regarded as a circular coil with very large number of turns. So, we can consider a solenoid up of elementary circular coils.
The difference is in their lengths. Solenoid has a longer length compared to its diameter. Also, the distance between its two loops is longer. While a coil has a bigger diameter and the distance between two loops in a coil is shorter compared to its diameter. Thus, we can say that a solenoid looks like a spring at its original or slightly elongated length and a coil as the same compressed spring. 

Nature of magnetic field inside a solenoid:

A solenoid produces an axial magnetic field forming south pole at one of its end and the north pole at the other end. The magnetic field lines inside the solenoid are in the form of parallel straight lines. Thus, the strength of the magnetic field is same at all points inside the solenoid. We can say that the magnetic field produced in the core of the solenoid is uniform.

Hence,a solenoid has uniform magnetic fields of lines that can be represented by equidistant, parallel straight lines pointing from south pole to north pole.